nisi fallor is another must-have expression. fallor is the Passive, First Person, Singular form of the verb fallo, fallere, fefelli, falsum. fallere means ‘to deceive’, ‘to lead astray’, ‘to cause to be mistaken’. Hence the passive fallor means something like ‘I am deceived’. And from there we have the very useful, nisi fallor, with which a Latin speaker can indicate their uncertainty about the statement that follows; ‘Unless I am mistaken….’.
I am quite certain, however, that this IS the same (eadem) woman that we saw on the roof of the bank.
Unless I am mistaken, she is the same woman that (ac) we saw on the roof of the bank. Without a doubt you are asking what she is doing on the roof of this building.