Well i think Tifa doesnt really forget what happened, sort of misplaced if you get what i mean.
Its like when you know a long word and you want to use that word but cant quite remember it, till you ask someone and they'll say it and your like "o yeah thats it" and same going for trying to remember someones face from tv, some one tells you and you remember instantly like you never forgot.
I think this is what happened to Tifa on ff7 when Cloud in Kalm tell his story about his last visit to Nibelheim. After he finishes, Tifa is unsure about what Cloud said, she knows somehow thats not what really happend but cant put her finger on what really did. If i remember correctly she thinks Cloud is not the real Cloud at some point. When they fall into the lifestream and she sees the truth, she instantly remembers.
Hopefully all that just made sense.