--- a/src/misc1.c
+++ b/src/misc1.c
@@ -2467,10 +2467,12 @@ skip_to_option_part(p)
 }
 
 /*
- * changed() is called when something in the current buffer is changed.
+ * Call this function when something in the current buffer is changed.
  *
  * Most often called through changed_bytes() and changed_lines(), which also
  * mark the area of the display to be redrawn.
+ *
+ * Careful: may trigger autocommands that reload the buffer.
  */
     void
 changed()
@@ -2545,6 +2547,7 @@ static void changed_common __ARGS((linen
  * - marks the windows on this buffer to be redisplayed
  * - marks the buffer changed by calling changed()
  * - invalidates cached values
+ * Careful: may trigger autocommands that reload the buffer.
  */
     void
 changed_bytes(lnum, col)
@@ -2658,6 +2661,7 @@ deleted_lines_mark(lnum, count)
  * below the changed lines (BEFORE the change).
  * When only inserting lines, "lnum" and "lnume" are equal.
  * Takes care of calling changed() and updating b_mod_*.
+ * Careful: may trigger autocommands that reload the buffer.
  */
     void
 changed_lines(lnum, col, lnume, xtra)
@@ -2725,6 +2729,11 @@ changed_lines_buf(buf, lnum, lnume, xtra
     }
 }
 
+/*
+ * Common code for when a change is was made.
+ * See changed_lines() for the arguments.
+ * Careful: may trigger autocommands that reload the buffer.
+ */
     static void
 changed_common(lnum, col, lnume, xtra)
     linenr_T	lnum;
@@ -2975,6 +2984,7 @@ check_status(buf)
  * Don't use emsg(), because it flushes the macro buffer.
  * If we have undone all changes b_changed will be FALSE, but "b_did_warn"
  * will be TRUE.
+ * Careful: may trigger autocommands that reload the buffer.
  */
     void
 change_warning(col)

--- a/src/undo.c
+++ b/src/undo.c
@@ -196,7 +196,7 @@ u_check_tree(u_header_T *uhp,
     }
 }
 
-    void
+    static void
 u_check(int newhead_may_be_NULL)
 {
     seen_b_u_newhead = 0;
@@ -331,6 +331,9 @@ undo_allowed()
     return TRUE;
 }
 
+/*
+ * Common code for various ways to save text before a change.
+ */
     static int
 u_savecommon(top, bot, newbot)
     linenr_T	top, bot;
@@ -385,7 +388,7 @@ u_savecommon(top, bot, newbot)
     size = bot - top - 1;
 
     /*
-     * if curbuf->b_u_synced == TRUE make a new header
+     * If curbuf->b_u_synced == TRUE make a new header.
      */
     if (curbuf->b_u_synced)
     {
@@ -1938,6 +1941,12 @@ u_doit(startcount)
 	u_oldcount = -1;
     while (count--)
     {
+	/* Do the change warning now, so that it triggers FileChangedRO when
+	 * needed.  This may cause the file to be reloaded, that must happen
+	 * before we do anything, because it may change curbuf->b_u_curhead
+	 * and more. */
+	change_warning(0);
+
 	if (undo_undoes)
 	{
 	    if (curbuf->b_u_curhead == NULL)		/* first undo */
@@ -2230,8 +2239,11 @@ undo_time(step, sec, file, absolute)
 	/*
 	 * First go up the tree as much as needed.
 	 */
-	for (;;)
+	while (!got_int)
 	{
+	    /* Do the change warning now, for the same reason as above. */
+	    change_warning(0);
+
 	    uhp = curbuf->b_u_curhead;
 	    if (uhp == NULL)
 		uhp = curbuf->b_u_newhead;
@@ -2248,9 +2260,15 @@ undo_time(step, sec, file, absolute)
 	/*
 	 * And now go down the tree (redo), branching off where needed.
 	 */
-	uhp = curbuf->b_u_curhead;
-	while (uhp != NULL)
+	while (!got_int)
 	{
+	    /* Do the change warning now, for the same reason as above. */
+	    change_warning(0);
+
+	    uhp = curbuf->b_u_curhead;
+	    if (uhp == NULL)
+		break;
+
 	    /* Go back to the first branch with a mark. */
 	    while (uhp->uh_alt_prev.ptr != NULL
 				     && uhp->uh_alt_prev.ptr->uh_walk == mark)
@@ -2349,6 +2367,12 @@ u_undoredo(undo)
     int		empty_buffer;		    /* buffer became empty */
     u_header_T	*curhead = curbuf->b_u_curhead;
 
+#ifdef FEAT_AUTOCMD
+    /* Don't want autocommands using the undo structures here, they are
+     * invalid till the end. */
+    block_autocmds();
+#endif
+
 #ifdef U_DEBUG
     u_check(FALSE);
 #endif
@@ -2378,6 +2402,9 @@ u_undoredo(undo)
 	if (top > curbuf->b_ml.ml_line_count || top >= bot
 				      || bot > curbuf->b_ml.ml_line_count + 1)
 	{
+#ifdef FEAT_AUTOCMD
+	    unblock_autocmds();
+#endif
 	    EMSG(_("E438: u_undo: line numbers wrong"));
 	    changed();		/* don't want UNCHANGED now */
 	    return;
@@ -2592,6 +2619,10 @@ u_undoredo(undo)
     /* The timestamp can be the same for multiple changes, just use the one of
      * the undone/redone change. */
     curbuf->b_u_time_cur = curhead->uh_time;
+
+#ifdef FEAT_AUTOCMD
+    unblock_autocmds();
+#endif
 #ifdef U_DEBUG
     u_check(FALSE);
 #endif