If a player glitches (via game lag) into a low ceiling that which they would fit under while sneaking at their altitude, they will have the sneaking hitbox and lowered view in which they can be to to y+.5 (where y+.2 is the normal hitbox collision) and can be replicated by using /tp higher than y+.2 up to y+.5.  The player model with still be straight up. 

Similarly glitching or using /tp into a ceiling higher than y+.5 up to y+1.4 will cause the player to crawl/swim in the air.