diff --git a/tests/test-lfs-largefiles.t b/tests/test-lfs-largefiles.t
--- a/tests/test-lfs-largefiles.t
+++ b/tests/test-lfs-largefiles.t
@@ -338,7 +338,10 @@ locally.
 No diffs when comparing merge and p1 that kept p1's changes.  Diff of lfs to
 largefiles no longer operates in standin files.
 
-  $ hg diff -r 2:3
+This `head -n 20` looks dumb (since we expect no output), but if something
+breaks you can get 1048576 lines of +y in the output, which takes a looooooong
+time to print.
+  $ hg diff -r 2:3 | head -n 20
   $ hg diff -r 2:6
   diff -r e989d0fa3764 -r 752e3a0d8488 large.bin
   --- a/large.bin	Thu Jan 01 00:00:00 1970 +0000