3. Touching Yourself Is Bad
The rules of time travel do vary from film to film, but one that I would probably not test out is the fate that one would suffer should one touch one's younger self. Ahem. This is the case in Timecop, where if you come into contact with the version of you from the past then because of the fact that two versions of the same matter cannot occupy the same space, you end up with a gloopy mess on the floor. The problem is that despite knowing this, you just know you would be so tempted to track down your past self, should you ever go back in time, and perhaps give yourself some advice on how to make more money or on why to avoid sleeping with Lisa from HR. With your excellent wisdom imparted you might high-five yourself and then implode. Avoid this grisly demise by remembering that touching yourself is bad and will cause a mess.
2. Your Friend Might Nail Your Mum
So there you are, leading a war against Skynet when the pesky little buggers go and send a Terminator back in time in order to kill your mother and stop you from ever being born, thus depriving the resistance of, apparently, the only person on the planet who could vaguely lead an army. Naturally, you would send back a trusted member of your team to go and sort out the whole thing, you would probably not expect that he would end up nailing your mum and somehow becoming your dad. I mean I would be pretty miffed if I sent someone to pick up my mum from the airport and they ducked into a Travel Lodge on the way back for a little afternoon delight, but at least that would not end up in a situation where my buddy becomes my actual dad. This point could also be used to illustrate that time travel makes absolutely no sense at all.
1. Just Kill Me Now...
Quite frankly, if we end up in a situation where Adam Sandler had control over all of time then then I would take it upon myself to somehow try and end all of existence, you know he would just make us watch Jack & Jill over and over again and humanity would be caught up in some sort of indescribably awful Groundhog Day.