Title says it all. Try "/execute @p ~0 ~0 ~0 kill @e[type=Item,c=1,r=5]", but without any items near the player. The command block's "Previous Output" reads "The entity UUID provided is in an invalid format" (which may be yet another bug in itself), however a comparator pointing out from the command block still receives power. It shouldn't, because the referenced command failed.

EDIT: I just realized this happens when using the new execute command. I'm assuming this is because the "execute" command technically succeeds, but the referenced command does not. Regardless, it should be fixed so that comparators detect the success state of the referenced command when "execute" succeeds.

That is, "execute" should return the success state of the command it ran (if "execute" itself was successful). Otherwise the success state of "execute" is almost meaningless.