I believed I would be able to detect a player that is within 10 blocks of a command block. I had a comparator pointing away from the block with redstone in front of it.

It didn't work though. I stepped on the command block and no redstone signal was given.

Reproducing this bug:
Place a command block with the command "testfor @p[r=10]"
Place a comparator pointing away from the command block
Place redstone in front of the comparator