go to disagree with you there, cockney steve, I've seen it happen a few times with strong braking during a landing...
In one case, a kid was lying across the mother's lap. we had a right time getting her to sit the child upright and put the armrest down (explaining that yes, it can move during landing and cause injury- not to mention that seatbelts don't work when they are loose and you are lying down)
Lo and behold we get the "crew prepare for landing" PA, I happen to look up and see the mother raise the armrest, and lay the kid back down. Despite telling her to correct it, she didn't and by this time it was semi-brace time so no more talking. (She had been warned)
We landed, quite a firm and fast one, and as soon as the guys hit reverse the armrest slammed down and clonked little Kevin on the head, eliciting a wail of pain and sent him crying for a good few minutes.
Not the last time I saw this happen either. I guess it depends on the aircraft make but I saw it more on older models. I guess the endless up/down of the armrests makes them easier to move. Newer aircraft you may notice have little pegs to stop them going backwards, but nothing for fowards, obviously.