This is pretty much a shot in the dark and googling has all but failed me.
http://content.ix2.net/arc/t-6317.html
This dude had the same problem, but I emailed him and he never got a solution. Basically, when CiscoACS runs it's checks in the night and on a handful of the Cataylst switches; for some reason, it issues the Q command to quit when it's not needed or something.
Anyways, it thinks the Q command is another user trying to log in, and our log files are full of invalid logins. Does anyone have any idea how to solve this?
This only happens on the 12-13 switches running the Cat OS