"your game may be modded" on unmodded clients

This is not a crash report.

The bug I am reporting is that when 13w38c crashes (which, although related, is a different issue), the launcher refuses to post the crash report through Hopper with the message “We think your game has been modded, and as such we can't accept this crash report.” However, neither my launcher nor my game and not even the server I was playing on during the crash were modded.

Why is this a problem?
Automatic submission of crash reports through the Hopper system make a lot of things easier, both for us people testing the snapshots and for you people at Mojang trying to fix the most annoying bugs.