The Day Enterprise Library Stopped Logging

Here's a fun one!  Our Enterprise Library exception handling logging mysteriously quit logging to the Application event log yesterday at 6:00 PM.  It was still logging to files, still handling all of the other exception handling rules, but just not logging to the event log.

After rolling back config changes, logging in as admin, rebooting (twice), pacing, etc. for about an hour, I found the obscure culprit. 

It turns out the default setting for the event log is to only keep 512K of data around, and in the event of more entries exceeding that, to delete entries older than seven days. 

 
Well, what if you have more than 512K of entries in the last seven days?  Yep ... it can't get rid of anything more, it can't exceed the 512K max, and therefore ... logging mysteriously quits.