Yeah, it's the most annoying thing for me, and I once (and only once) flew to Perth on Jetstar, not only did the squirmer in front near hobble me, the bloke alongside got airsick beside me, so struggling to walk, LHS shirt and shorts covered in vomit, asked if they could find a spare seat for me- answer was no, and please return to your seat as you are obstructing the walkway! Thanks for that!
The time I ended up with a dislocated kneecap was a bit different, we all had boarded in Karratha 20 minutes earlier, then had to wait for a offshore crew's chopper to arrive and connect, and as they got on, the lil fella jumped in the seat and immediately dropped it back with no warning, he must have been a bit tuckered out from his previous flight - POP, and instinct for me was a loud yell of ##%#, a slap and pushing the seat forward hard to get his weight off my leg. Naturally most of the plane wondered what the disturbance was, he turned around and started to vent, the guys beside me were giving it to him, and then I stood up in obvious pain, and as he registered I was at least a foot taller and 50 kg heavier he started to slink back down. Painful few hours until I hit Perth to get it looked at, but karma is great, 2 months later I arrived at his rig to teach them about risk mitigation and identifying hazards - naturally I had a very real scenario involving said individual much to his workmates enjoyment!