From time to time, one of you will stuff up. It's inevitable (unless you are 7ft tall and blue)
I'd watch how you react to your other persons stuff up, otherwise a funny phenomenon will occur.
It won't stop anyone making mistakes but it may make them stop telling you when they have done so or perhaps make them just go away altogether.
If there is real remorse and a promise to attempt to never do such a stuff up again,and you still want them around, sometimes that truly is enough.
Truly..
