When you enter a password and it's correct the response is practically instantaneous (i.e. the log on process).
When you enter an incorrect password (accidently, forgotten, etc) however, it takes a while (10-30 seconds) before it responds that the password was incorrect.
Why does it take so long (relatively) to say "incorrect password"?
This has always bugged me about entering incorrect passwords on Windows and linux (real and VM); I'm not sure about mac OSX as I can't remember if it's the same, been awhile since I last used a mac.
EDIT: for the sake of duplication, I'm asking in the context of a user logging on to the system at the physical computer rather than through ssh which could conceivably use somewhat different mechanisms to log in/validate credentials.