When I went back to Aeris' church later in the game (using the key I got in Mideel), I saw Aeris in a flash and she immediately disappeared.

At first I thought it was my imagination, but when I left Midgar and then came back it happened again!
I thought it was a bug in my copy of FF or something, but a friend of mine had the same.

So, everyone who CAN do it, please look if you have that too.

A bug? A secret?

I don't know, can anyone of you tell me about this?