(This occurred when I was making a map that required the player to 'crawl' in a one block high hole.)

Whenever a player is in a FallingSand entity resting on a Piston extension, they are unable to be detected via command block.

To achieve, dig a hole, and in that hole type

/setblock ~ ~ ~ minecraft:piston_extention

then fly up a little, and type

/summon FallingSand ~ ~ ~ {Time:1}

Now make a testfor command block, searching for a player in that block.
Make sure there is a radius set. When I tested it, if I just searched for @a it worked perfectly fine.