Testforblock command continues to output to comparators, even when the block it's testing for is gone.

Cannot confirm.

Getting with block-to-test

[23:17:17] Successfully found the block at 572,71,246.

without block-to-test

[23:18:00] The block at 572,71,246 is tile.air.name (expected: Erde).

Did you repower the command block after the block-to-test is gone ?

Yes. It if makes a difference, it's on a server.

Can also not confirm on server / client b

Is this still a concern in the latest Minecraft version 13w48b? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.

Are you sending a redstone signal after removing the block?

No I wasn't.

Sorry and thanks.

So is this a bug now or isn't it? because i am still getting the same result in the latest snapshot 14w04b...

I've got the same problem, on the Server i'm playing the commandblock still powers the comperator even when he's not powerd by a clock, but in singleplayer it works.