Invalid User in RADIUS Logs

Hi, Will a wrong password also will sometimes log 'invalid user' in the RADIUS log file? Coz recently i get a lot of these in the log file. But the users are actually valid and was able to authenticate successfully later on. Just want to get possible reasons on what could be causing RADIUS to log that. I'm also suspecting perhaps it was not able to lookup/connect to MySQL db at that time. Would that be possible cause also? Thanks, Det