Consider the following test.diff file:

--- a
+++ b
@@ -1,1 +1,1 @@
-0123456789
+.0123456789.0123456789xx

--- a
+++ b
@@ -1,1 +1,1 @@
-0123456789
+.0123456789.0123456789

When I open it with "emacs --no-site-file -Q test.diff", the
highlighting of the first diff is incorrect: the following
characters are highlighted:

-0123456789
+.0123456789.0123456789xx
 ^^^^^^^    ^^^^      ^^^

The highlighting in the second diff is correct:

-0123456789
+.0123456789.0123456789
 ^^^^^^^^^^^^

I've also attached a screenshot.