@Joe 35 :
"...You enter "Password_4".
System sees last digit is a number, replaces that number with n-1, generates hash result (for Password_3 in this example) and sees if it is a match with existing password hash. If it is, slapped wrist."
A nice solution. Though I admit, I'd probably just switch to PasswordJan, PasswordFeb, etc.
It's not a good solution though is it? It reveals that the password hashes aren't salted which is crap, and if they were then this wouldn't work.