We are only human, and we do miss things. If the FA's had seen the carrier, and you were seated in the exit row, you would have been moved. Some of the soft-sided carriers are hard to distinguish from regular carry-on baggage. I think that part of the reason of putting pet info on our paperwork is to make sure that there are no pets in exit/bulkhead rows. If you were in the row behind the exit row, then that is acceptable.