I never really used summons at all so this is something I just discovered while rewatching a Let's Play of FFX.
We all know the other two characters in the party have to hightail it out of there when Yuna summons so the aeon can take their place in the battle. But if the character is KO'ed and lying there, he/she will still get up and run away and this causes the game to glitch or bug out or whatever you want to call it for a second.
Let's Play Final Fantasy X Ep 32 - YouTube
Let's Play Final Fantasy X Ep 41 - YouTube
I think it looks pretty funny.