In the Shinra building, right before you climb up the vent in the bathroom to eavesdrop on the company's conversation, you can flush the toilet you climb onto and I did that before I climbed.
My brother said to me, "I bet you are the only person to play this game and actually flush that toilet. "
I did it about 5xs before moving on... lol
"The world needs a new hero" -Genesis, Crisis Core: Final Fantasy VII
"Come and try" - Sephiroth, Crisis Core: Final Fantasy VII