definitely depends on what the person screwed up on.
if they screwed up by cheating then kick them out to the curb. i don't believe in people in that nonsense of "in the heat of the moment", being unfaithful is not something that occurs in a moment, the person has a choice and plenty of time to consider the consequences, and clearly chose to 1. not consider or 2. not care about the consequences. both should not be forgiven nor forgotten.
on the other hand, if ...say...a guy "screws up" on forgetting their g/f's special occassion, such as her bday, their anniversary, etc. then the guy should clearly be given a second chance, (if not a third and a fourth and so on )