What stroke mistakes most often get young swimmers disqualified?

Question

Accepted Answer

These are the rules young swimmers most often break (Article 101):

Stroke	Key rules
Breaststroke	Arms and legs move together (no alternating). The cycle is one arm pull then one kick. Feet turn outward on the kick (no flutter or scissor kick). Only one butterfly kick is allowed after the start and each turn. Two hands touch together at every turn and the finish.
Butterfly	Both arms recover over the water and pull together. The legs kick together (no flutter or breaststroke kick). Two-hand touch at every turn and the finish.
Backstroke	Stay on the back except during the turn, and be back on the back when leaving the wall. Touch the wall on your back at the finish.
Freestyle	Any stroke is allowed. Standing on the bottom isn't a disqualification, but walking or pushing off the bottom is.
Individual medley	The order is butterfly, backstroke, breaststroke, freestyle. Each changeover follows the finish rule for the stroke just completed.

In every stroke, swimmers must finish in the lane they started in, and pulling on the lane ropes isn't allowed. Standing on the bottom in any stroke other than freestyle is a disqualification.