diff --git a/diff-lib.c b/diff-lib.c
index 0c74ef5cbe..adf1c5fdee 100644
--- a/diff-lib.c
+++ b/diff-lib.c
@@ -383,7 +383,7 @@ static inline void skip_same_name(struct cache_entry *ce, struct unpack_trees_op
  * For diffing, the index is more important, and we only have a
  * single tree.
  *
- * We're supposed to return how many index entries we want to skip.
+ * We're supposed to advance o->pos to skip what we have already processed.
  *
  * This wrapper makes it all more readable, and takes care of all
  * the fairly complex unpack_trees() semantic requirements, including
diff --git a/unpack-trees.c b/unpack-trees.c
index 157d5d001f..dd5999c356 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -277,6 +277,17 @@ static int unpack_nondirectories(int n, unsigned long mask,
 	return 0;
 }
 
+static int unpack_failed(struct unpack_trees_options *o, const char *message)
+{
+	discard_index(&o->result);
+	if (!o->gently) {
+		if (message)
+			return error("%s", message);
+		return -1;
+	}
+	return -1;
+}
+
 static int unpack_callback(int n, unsigned long mask, unsigned long dirmask, struct name_entry *names, struct traverse_info *info)
 {
 	struct cache_entry *src[MAX_UNPACK_TREES + 1] = { NULL, };
@@ -294,7 +305,7 @@ static int unpack_callback(int n, unsigned long mask, unsigned long dirmask, str
 			int cmp = compare_entry(ce, info, p);
 			if (cmp < 0) {
 				if (unpack_index_entry(ce, o) < 0)
-					return -1;
+					return unpack_failed(o, NULL);
 				continue;
 			}
 			if (!cmp) {
@@ -352,17 +363,6 @@ static int unpack_callback(int n, unsigned long mask, unsigned long dirmask, str
 	return mask;
 }
 
-static int unpack_failed(struct unpack_trees_options *o, const char *message)
-{
-	discard_index(&o->result);
-	if (!o->gently) {
-		if (message)
-			return error("%s", message);
-		return -1;
-	}
-	return -1;
-}
-
 /*
  * N-way merge "len" trees.  Returns 0 on success, -1 on failure to manipulate the
  * resulting index, -2 on failure to reflect the changes to the work tree.
@@ -617,7 +617,7 @@ static int verify_absent(struct cache_entry *ce, const char *action,
 			 * found "foo/." in the working tree.
 			 * This is tricky -- if we have modified
 			 * files that are in "foo/" we would lose
-			 * it.
+			 * them.
 			 */
 			ret = verify_clean_subdirectory(ce, action, o);
 			if (ret < 0)