The best trolls go unnoticed, like an internet ninja, assassinating your targets with rage.  Or at least a strange awkward feeling.  It's even better if you can disguise it as an act of kindness, instead of a stupid question or something that will be raged at regardless if it's a troll or not.

Farting in an elevator is classic.  Everyone smells it, but nobody knows who did it.  Nobody wants to ask who did it either, that would just be awkward, really.  Unless you're a troll.  But making a big fuss inside of a crowded elevator, yelling out for the person who passed the gas (while it was actually you) is not good enough.  Here's what you do to silently troll random people anywhere.

The door opener:  Open a door for someone across the room and watch as they try to go faster to reach it, in order to not make you feel uncomfortable for standing there... if only they knew.

The Pointer: Point out something.  Anything.  It doesn't have to be there.  Actually, it's better if it's not.  A plane in the sky.  A bird in a tree.  Watch as they strain to look for it.  Then describe it, and ask them what they think.  Soon, they will start agreeing with everything you say, because if they say otherwise they might make you think you're seeing something that isn't there.

Oh silly little sheep, your feelings of consideration only backfire on you.  Trolls: 1 Normal People: 0