diff --git a/cache/filecache/filecache_pruner.go b/cache/filecache/filecache_pruner.go
index d6322cb70..cd95766be 100644
--- a/cache/filecache/filecache_pruner.go
+++ b/cache/filecache/filecache_pruner.go
@@ -17,6 +17,7 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"strings"
 
 	"github.com/gohugoio/hugo/common/herrors"
 	"github.com/gohugoio/hugo/hugofs"
@@ -59,6 +60,29 @@ func (c *Cache) Prune(force bool) (int, error) {
 
 	counter := 0
 
+	seen := c.entryLocker.seen
+	seenByLower := make(map[string]string, seen.Len())
+	for id := range seen.All() {
+		seenByLower[strings.ToLower(id)] = id
+	}
+
+	// Names on disk matching a used cache key except for the case, and the used keys
+	// actually walked. See the note about case-insensitive filesystems below.
+	var candidates map[string]string
+	visited := make(map[string]bool, seen.Len())
+
+	remove := func(name string) error {
+		err := c.Fs.Remove(name)
+		if err == nil {
+			counter++
+			return nil
+		}
+		if !herrors.IsNotExist(err) {
+			return err
+		}
+		return nil
+	}
+
 	err := afero.Walk(c.Fs, "", func(name string, info os.FileInfo, err error) error {
 		if info == nil {
 			return nil
@@ -93,27 +117,44 @@ func (c *Cache) Prune(force bool) (int, error) {
 
 		shouldRemove := force || c.isExpired(info.ModTime())
 
-		if !shouldRemove && c.entryLocker.seen.Len() > 0 {
-			// Remove it if it's not been touched/used in the last build.
-			shouldRemove = !c.entryLocker.seen.Has(name)
+		if seen.Has(name) {
+			visited[name] = true
+		} else if !shouldRemove && seen.Len() > 0 {
+			if id, found := seenByLower[strings.ToLower(name)]; found {
+				// On case-insensitive filesystems this is the same file as id; e.g. an
+				// entry created before Hugo started lowercasing the content paths in
+				// v0.123 (content/MyBundle => _gen/images/MyBundle). Decided once the
+				// walk is done: if id is walked too, they are distinct files and this
+				// one is stale. See issue 15101.
+				if candidates == nil {
+					candidates = make(map[string]string)
+				}
+				candidates[name] = id
+			} else {
+				// Remove it if it's not been touched/used in the last build.
+				shouldRemove = true
+			}
 		}
 
 		if shouldRemove {
-			err := c.Fs.Remove(name)
-			if err == nil {
-				counter++
-			}
-
-			if err != nil && !herrors.IsNotExist(err) {
-				return err
-			}
-
+			return remove(name)
 		}
 
 		return nil
 	})
+	if err != nil {
+		return counter, err
+	}
 
-	return counter, err
+	for name, id := range candidates {
+		if visited[id] {
+			if err := remove(name); err != nil {
+				return counter, err
+			}
+		}
+	}
+
+	return counter, nil
 }
 
 func (c *Cache) pruneRootDirs(force bool) (int, error) {
diff --git a/cache/filecache/filecache_pruner_test.go b/cache/filecache/filecache_pruner_test.go
index 117ff3430..dc01e417a 100644
--- a/cache/filecache/filecache_pruner_test.go
+++ b/cache/filecache/filecache_pruner_test.go
@@ -20,11 +20,72 @@ import (
 	"time"
 
 	"github.com/gohugoio/hugo/cache/filecache"
+	"github.com/gohugoio/hugo/htesting"
 	"github.com/spf13/afero"
 
 	qt "github.com/frankban/quicktest"
 )
 
+// A cache entry created before Hugo started lowercasing content paths in v0.123
+// (e.g. _gen/images/MyBundle) is on a case-insensitive filesystem the same file as
+// the lowercased cache key used today, and must not be pruned.
+// See issue 15101.
+func TestPruneCacheEntryWithOtherCase(t *testing.T) {
+	t.Parallel()
+	c := qt.New(t)
+
+	dir := t.TempDir()
+	if isCaseInsensitive, err := htesting.IsCaseInsensitiveFs(dir); err != nil {
+		t.Fatal(err)
+	} else if !isCaseInsensitive {
+		t.Skip("skip test on case-sensitive filesystem")
+	}
+
+	fs := afero.NewBasePathFs(afero.NewOsFs(), dir)
+	newCache := func() *filecache.Cache {
+		return filecache.NewCache(fs, filecache.FileCacheConfig{Dir: "cache", MaxAge: -1})
+	}
+
+	c.Assert(newCache().SetBytes("MyBundle/i1", []byte("abc")), qt.IsNil)
+
+	cache := newCache()
+	_, b, err := cache.GetOrCreateBytes("mybundle/i1", func() ([]byte, error) {
+		return []byte("def"), nil
+	})
+	c.Assert(err, qt.IsNil)
+	c.Assert(string(b), qt.Equals, "abc")
+
+	count, err := cache.Prune(false)
+	c.Assert(err, qt.IsNil)
+	c.Assert(count, qt.Equals, 0)
+	c.Assert(cache.GetString("MyBundle/i1"), qt.Equals, "abc")
+}
+
+// On a case-sensitive filesystem the entries above are distinct files,
+// and the one not used in this build should be pruned.
+func TestPruneCacheEntryWithOtherCaseCaseSensitiveFs(t *testing.T) {
+	t.Parallel()
+	c := qt.New(t)
+
+	fs := afero.NewMemMapFs()
+	cache := filecache.NewCache(fs, filecache.FileCacheConfig{Dir: "cache", MaxAge: -1})
+
+	c.Assert(cache.SetBytes("MyBundle/i1", []byte("abc")), qt.IsNil)
+
+	cache = filecache.NewCache(fs, filecache.FileCacheConfig{Dir: "cache", MaxAge: -1})
+	_, b, err := cache.GetOrCreateBytes("mybundle/i1", func() ([]byte, error) {
+		return []byte("def"), nil
+	})
+	c.Assert(err, qt.IsNil)
+	c.Assert(string(b), qt.Equals, "def")
+
+	count, err := cache.Prune(false)
+	c.Assert(err, qt.IsNil)
+	c.Assert(count, qt.Equals, 1)
+	c.Assert(cache.GetString("MyBundle/i1"), qt.Equals, "")
+	c.Assert(cache.GetString("mybundle/i1"), qt.Equals, "def")
+}
+
 func TestPrune(t *testing.T) {
 	t.Parallel()
 
diff --git a/htesting/test_helpers.go b/htesting/test_helpers.go
index 384695df4..46f6bd9ec 100644
--- a/htesting/test_helpers.go
+++ b/htesting/test_helpers.go
@@ -16,6 +16,7 @@ package htesting
 import (
 	"math/rand"
 	"os"
+	"path/filepath"
 	"regexp"
 	"runtime"
 	"strconv"
@@ -58,6 +59,27 @@ func CreateTempDir(fs afero.Fs, prefix string) (string, func(), error) {
 	return tempDir, func() { fs.RemoveAll(tempDir) }, nil
 }
 
+// IsCaseInsensitiveFs reports whether dir lives on a case-insensitive filesystem
+// (e.g. the default on macOS and Windows).
+func IsCaseInsensitiveFs(dir string) (bool, error) {
+	f, err := os.CreateTemp(dir, "case-*.tmp")
+	if err != nil {
+		return false, err
+	}
+	f.Close()
+	defer os.Remove(f.Name())
+
+	_, err = os.Stat(filepath.Join(dir, strings.ToUpper(filepath.Base(f.Name()))))
+	if err != nil {
+		if os.IsNotExist(err) {
+			return false, nil
+		}
+		return false, err
+	}
+
+	return true, nil
+}
+
 // BailOut panics with a stack trace after the given duration. Useful for
 // hanging tests.
 func BailOut(after time.Duration) {
diff --git a/main_test.go b/main_test.go
index 12e708dc0..8cea78818 100644
--- a/main_test.go
+++ b/main_test.go
@@ -15,6 +15,7 @@ package main
 
 import (
 	"bytes"
+	"encoding/base64"
 	"encoding/json"
 	"fmt"
 	"io"
@@ -27,6 +28,7 @@ import (
 	"runtime"
 	"strconv"
 	"strings"
+	"sync"
 	"testing"
 	"time"
 
@@ -78,10 +80,22 @@ func TestMain(m *testing.M) {
 	})
 }
 
+var isCaseInsensitiveFs = sync.OnceValues(func() (bool, error) {
+	return htesting.IsCaseInsensitiveFs(os.TempDir())
+})
+
 var commonTestScriptsParam = testscript.Params{
 	Setup: func(env *testscript.Env) error {
 		return testSetupFunc()(env)
 	},
+	Condition: func(cond string) (bool, error) {
+		switch cond {
+		case "caseinsensitivefs":
+			return isCaseInsensitiveFs()
+		default:
+			return false, fmt.Errorf("unknown condition %q", cond)
+		}
+	},
 	Cmds: map[string]func(ts *testscript.TestScript, neg bool, args []string){
 		// log prints to stderr.
 		"log": func(ts *testscript.TestScript, neg bool, args []string) {
@@ -252,6 +266,34 @@ var commonTestScriptsParam = testscript.Params{
 				ts.Fatalf("failed to create symlink: %v", err)
 			}
 		},
+		// base64decode decodes a base64-encoded file into a binary file.
+		"base64decode": func(ts *testscript.TestScript, neg bool, args []string) {
+			if len(args) != 2 {
+				ts.Fatalf("usage: base64decode src_base64_file dest_binary_file")
+			}
+
+			// Resolve paths relative to the test sandbox's current directory
+			src := ts.MkAbs(args[0])
+			dest := ts.MkAbs(args[1])
+
+			// Read the base64 text
+			base64Data, err := os.ReadFile(src)
+			if err != nil {
+				ts.Fatalf("failed to read base64 file: %v", err)
+			}
+
+			// Decode base64 back to raw bytes.
+			binaryBytes, err := base64.StdEncoding.DecodeString(strings.TrimSpace(string(base64Data)))
+			if err != nil {
+				ts.Fatalf("failed to decode base64 data: %v", err)
+			}
+
+			// Write the binary file.
+			err = os.WriteFile(dest, binaryBytes, 0o644)
+			if err != nil {
+				ts.Fatalf("failed to write binary file: %v", err)
+			}
+		},
 
 		// httpget checks that a HTTP resource's body matches (if it compiles as a regexp) or contains all of the strings given as arguments.
 		"httpget": func(ts *testscript.TestScript, neg bool, args []string) {
diff --git a/testscripts/commands/hugo_gc_mixed_case_cache_dir.txt b/testscripts/commands/hugo_gc_mixed_case_cache_dir.txt
new file mode 100644
index 000000000..7c7af1373
--- /dev/null
+++ b/testscripts/commands/hugo_gc_mixed_case_cache_dir.txt
@@ -0,0 +1,38 @@
+# In Hugo v0.123 we started lowercasing the content paths, but on case-insensitive
+# filesystems existing caches keep their mixed-case dir names (e.g. MyBundle).
+# The pruner would then consider those entries unused and remove them on every
+# hugo --gc. See issue 15101.
+
+[!caseinsensitivefs] skip
+
+base64decode pixel.png.txt content/MyBundle/pix.png
+
+hugo
+
+stdout 'Processed images │  1 '
+tree resources/_gen
+stdout 'pix_hu_a354833fd576551d.png'
+
+# Simulate a cache created by an older Hugo version.
+mv resources/_gen/images/mybundle resources/_gen/images/MyBundle
+
+hugo --gc
+stdout 'Cleaned          │  0 '
+
+tree resources/_gen
+stdout 'pix_hu_a354833fd576551d.png'
+
+-- hugo.toml --
+disableKinds = ["term", "taxonomy", "home"]
+-- layouts/all.html --
+All.
+-- layouts/page.html --
+{{ $pix := .Resources.Get "pix.png" }}
+{{ $resized := $pix.Resize "10x10" }}
+Resized: {{ $resized.RelPermalink }}
+-- content/MyBundle/index.md --
+---
+title: MyBundle
+---
+-- pixel.png.txt --
+iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNkYPhfDwAChwGA60e6kgAAAABJRU5ErkJggg==