diff -r ad87e19854a6 -r 235443668bea mercurial/mdiff.py
--- a/mercurial/mdiff.py	Mon Jun 13 13:43:48 2005 -0800
+++ b/mercurial/mdiff.py	Mon Jun 13 14:53:33 2005 -0800
@@ -50,7 +50,7 @@
         am, bm, = la, lb
 
         # walk over matching lines
-        while lb < lenb and la < lenb and a[la] == b[lb] :
+        while lb < lenb and la < lena and a[la] == b[lb] :
             la += 1
             lb += 1