diff --git a/TODO b/TODO
index 9488fea..f8dc34a 100644
--- a/TODO
+++ b/TODO
@@ -2,3 +2,8 @@ Add --include option (opposite of --exclude).
 
 Look into sdiff improvement here:
 http://www.pkix.net/~chuck/sdiff2.diff
+
+Propagate stderr from subprocess so that diff3 does
+a better job of explaining _why_:
+  > $ diff3 /bin/sh /bin/false /bin/mv
+  > diff3: subsidiary program `diff' failed (exit status 2)