One amusing moment I had was someone recently taking the absolute mickey using psyker abilities. I'd created a custom weapon for a librarian to use in my game, a relic tier force sword capable of opening a gate of infinity a couple of times per session at a fixed PR of 4 and only good for personal use. The group were then about to enter a tank battle, pitching their tanks versus the great enemy. The Librarian went with an Apothecary in a Predator, while the rest took a Land Raider and Air support another way.
So the predator got separated and took a bit of a hammering from enemies, and was about to get a lot worse when, rolling over the horizon, came an enemy land raider. In a moment of pure genius which to this day I have not found anything to beat, the librarian opted to use gate of Infinity from their Land Raider and apparate inside the enemy Land Raider. Since it doesn't technically say you need line of sight or anything to use the original power, I let him get away with it because the group were laughing too hard for me to say no. So he appears inside a Chaos Land Raider, slaughters the heretical crew and then commandeers it. They were not able to get a message out either that their tank had been stolen so he was quite happy rolling the other way back through enemy lines and nobody batted an eyelid!
It then got worse when he finally got rumbled by another enemy tank which opened fire. He used it again to jump into a second tank, took out the crew but this time rather than move on, he opened fire on one of the enemy armour, waited for them to start returning fire and used the final charge of the blade to get out of the vehicle entirely allowing them to blow up an undriven vehicle!