Tray tables down during take-off and landing is a big no-no I agree, and it should be mentioned. I apologise if I misread the first post.
The differential braking effect occasionally caused by reverse thrust can be particularly dangerous while landing on a soaked runway, especially in a crosswind. I have veered off centreline quite significantly once, and a colleague once took out the edge lights and wrote off 4 mainwheel tyres.
S.