Bar/bas mitzvah. If it’s clear that it’s not real there may be a heter, I’m not sure. If we’re talking really after the fact – like the girl grew up, got married, had a kid, and realized “yoish my kid might be a mamzer” – I am sure a rav would use this angle to say there is no problem whatsoever, but I don’t know the sugya well enough to be more specific.
