I once tried to make a booking at AY.com and it didn't go through because one digit of my then-AA-FF# number was wrong. So the system detected that and I believe it was actually able to tell me that the name and FF# didn't match.

And I know of a father and son who have the same first (and last) name. They have a joint FF# account Need to make sure not to fly on the same day, though...