diff --git a/lfs.c b/lfs.c
index d50335f2..bae403c5 100644
--- a/lfs.c
+++ b/lfs.c
@@ -9523,6 +9523,7 @@ static inline bool lfsr_path_isdir(const char *path) {
 // - LFS_ERR_NOENT, lfsr_path_islast(path)  => file not found
 // - LFS_ERR_NOENT, !lfsr_path_islast(path) => parent not found
 // - LFS_ERR_NOTDIR                         => parent not a dir
+// - LFS_ERR_NOTSUP                         => parent of unknown type
 //
 // if not found, mdir_/did_ will at least be set up with what should be
 // the parent
@@ -9603,8 +9604,10 @@ static int lfsr_mtree_pathlookup(lfs_t *lfs, const char **path,
         // only continue if we hit a directory
         if (tag != LFSR_TAG_DIR) {
             return (tag == LFSR_TAG_STICKYNOTE)
-                    ? LFS_ERR_NOENT
-                    : LFS_ERR_NOTDIR;
+                        ? LFS_ERR_NOENT
+                    : (lfsr_tag_isunknown(tag))
+                        ? LFS_ERR_NOTSUP
+                        : LFS_ERR_NOTDIR;
         }
 
         // read the next did from the mdir if this is not the root
@@ -10606,6 +10609,10 @@ int lfsr_remove(lfs_t *lfs, const char *path) {
     if (tag == LFSR_TAG_STICKYNOTE) {
         return LFS_ERR_NOENT;
     }
+    // we can't remove unknown types or else we may leak resources
+    if (lfsr_tag_isunknown(tag)) {
+        return LFS_ERR_NOTSUP;
+    }
 
     // trying to remove the root dir?
     if (mdir.mid == -1) {
@@ -10725,6 +10732,10 @@ int lfsr_rename(lfs_t *lfs, const char *old_path, const char *new_path) {
     if (old_tag == LFSR_TAG_STICKYNOTE) {
         return LFS_ERR_NOENT;
     }
+    // we can't rename unknown types or else we may leak resources
+    if (lfsr_tag_isunknown(old_tag)) {
+        return LFS_ERR_NOTSUP;
+    }
 
     // trying to rename the root?
     if (old_mdir.mid == -1) {
@@ -10740,6 +10751,10 @@ int lfsr_rename(lfs_t *lfs, const char *old_path, const char *new_path) {
     if (err && !(err == LFS_ERR_NOENT && lfsr_path_islast(new_path))) {
         return err;
     }
+    // we can't rename unknown types or else we may leak resources
+    if (lfsr_tag_isunknown(new_tag)) {
+        return LFS_ERR_NOTSUP;
+    }
     // already exists?
     bool exists = (err != LFS_ERR_NOENT);
 
@@ -11616,7 +11631,9 @@ int lfsr_file_opencfg(lfs_t *lfs, lfsr_file_t *file,
 
         // wrong type?
         if (tag != LFSR_TAG_REG) {
-            return LFS_ERR_ISDIR;
+            return (tag == LFSR_TAG_DIR)
+                    ? LFS_ERR_ISDIR
+                    : LFS_ERR_NOTSUP;
         }
     }
 
diff --git a/tests/test_mount.toml b/tests/test_mount.toml
index e79f04ec..2b285ae7 100644
--- a/tests/test_mount.toml
+++ b/tests/test_mount.toml
@@ -902,3 +902,96 @@ code = '''
     lfsr_mount(&lfs, LFS_M_RDONLY, CFG) => LFS_ERR_NOTSUP;
 '''
 
+# test what happens if we find an unknown file type
+[cases.test_mount_incompat_unknown_type]
+in = 'lfs.c'
+code = '''
+    // create a superblock
+    lfs_t lfs;
+    lfsr_format(&lfs, LFS_F_RDWR, CFG) => 0;
+
+    // create some files
+    lfsr_mount(&lfs, LFS_M_RDWR, CFG) => 0;
+    lfsr_file_t file;
+    lfsr_file_open(&lfs, &file, "a",
+            LFS_O_WRONLY | LFS_O_CREAT | LFS_O_EXCL) => 0;
+    lfsr_file_write(&lfs, &file,
+            "hi a!", strlen("hi a!")) => strlen("hi a!");
+    lfsr_file_close(&lfs, &file) => 0;
+    lfsr_file_open(&lfs, &file, "b",
+            LFS_O_WRONLY | LFS_O_CREAT | LFS_O_EXCL) => 0;
+    lfsr_file_write(&lfs, &file,
+            "oh no!", strlen("oh no!")) => strlen("oh no!");
+    lfsr_file_close(&lfs, &file) => 0;
+    lfsr_file_open(&lfs, &file, "c",
+            LFS_O_WRONLY | LFS_O_CREAT | LFS_O_EXCL) => 0;
+    lfsr_file_write(&lfs, &file,
+            "hi c!", strlen("hi c!")) => strlen("hi c!");
+    lfsr_file_close(&lfs, &file) => 0;
+    lfsr_unmount(&lfs) => 0;
+
+    // change a file's type to something unknown
+    lfsr_mount(&lfs, LFS_M_RDWR, CFG) => 0;
+    const char *path = "b";
+    lfsr_mdir_t mdir;
+    lfsr_did_t did;
+    lfsr_mtree_pathlookup(&lfs, &path,
+            &mdir, NULL, &did) => 0;
+    lfsr_mdir_commit(&lfs, &mdir, LFSR_RATS(
+            LFSR_RAT_NAME(
+                LFSR_TAG_SUB | (LFSR_TAG_NAME + 0x13), 0,
+                did, path, lfsr_path_namelen(path)))) => 0;
+    lfsr_unmount(&lfs) => 0;
+
+    // mount
+    lfsr_mount(&lfs, LFS_M_RDWR, CFG) => 0;
+
+    // our file should appear as an unknown type
+    struct lfs_info info;
+    lfsr_stat(&lfs, "b", &info) => 0;
+    assert(strcmp(info.name, "b") == 0);
+    assert(info.type == 0x13);
+    assert(info.size == 0);
+
+    lfsr_dir_t dir;
+    lfsr_dir_open(&lfs, &dir, "/") => 0;
+    lfsr_dir_read(&lfs, &dir, &info) => 0;
+    assert(strcmp(info.name, ".") == 0);
+    assert(info.type == LFS_TYPE_DIR);
+    assert(info.size == 0);
+    lfsr_dir_read(&lfs, &dir, &info) => 0;
+    assert(strcmp(info.name, "..") == 0);
+    assert(info.type == LFS_TYPE_DIR);
+    assert(info.size == 0);
+    lfsr_dir_read(&lfs, &dir, &info) => 0;
+    assert(strcmp(info.name, "a") == 0);
+    assert(info.type == LFS_TYPE_REG);
+    assert(info.size == strlen("hi a!"));
+    lfsr_dir_read(&lfs, &dir, &info) => 0;
+    assert(strcmp(info.name, "b") == 0);
+    assert(info.type == 0x13);
+    assert(info.size == 0);
+    lfsr_dir_read(&lfs, &dir, &info) => 0;
+    assert(strcmp(info.name, "c") == 0);
+    assert(info.type == LFS_TYPE_REG);
+    assert(info.size == strlen("hi c!"));
+    lfsr_dir_read(&lfs, &dir, &info) => LFS_ERR_NOENT;
+    lfsr_dir_close(&lfs, &dir) => 0;
+
+    // removing/renaming unknown files should return NOTSUP, we could
+    // remove the metadata entry, but we would probably leak stuff
+    lfsr_remove(&lfs, "b") => LFS_ERR_NOTSUP;
+    lfsr_rename(&lfs, "b", "d") => LFS_ERR_NOTSUP;
+    lfsr_rename(&lfs, "b", "c") => LFS_ERR_NOTSUP;
+    lfsr_rename(&lfs, "a", "b") => LFS_ERR_NOTSUP;
+
+    lfsr_file_open(&lfs, &file, "b",
+            LFS_O_RDONLY) => LFS_ERR_NOTSUP;
+    lfsr_file_open(&lfs, &file, "b",
+            LFS_O_WRONLY | LFS_O_CREAT) => LFS_ERR_NOTSUP;
+
+    lfsr_mkdir(&lfs, "b") => LFS_ERR_EXIST;
+
+    lfsr_unmount(&lfs) => 0;
+'''
+