Reasons for "MS-CHAP2-Response is incorrect"

Hi all! The 1st level support group here has asked me to clarify the reasons behind authentication errors where the message is "mschap: MS-CHAP2-Response is incorrect" so they can better point the end users in the right direction. The obvious one is of course "wrong password", another one is "mucking about with the Username in the RADIUS server" (which I don't do). What other reasons are there for this error type to occur? Grüße, Sven.