For the last two days, I have been checking my SQL Server error logs and this is what I find common on all the SQL Servers, with Audit Login options ON: Login failed for 'A', Login failed for 'B', Login Failed for 'C'..all the way to the last login.
For each login defined in the SQL Server, I'm finding that the error message is getting generated 18 times. My guess is that someone is trying to run some kind of code.
Make sure you don't have any easily guessable accounts, and DEFINITELY, - change your SA password. This is not a bug (unless you don't have the latest security patch from M$, in which case it is and has been fixed). Whatch out, someone is after your server.