If you power a block of TNT and a piston pushing it at the same time, it creates a glitchy phantom tnt block, that disappears as soon as the real TNT explodes. Image: https://twitter.com/Creeper0108/status/230531701628104704/photo/1 (Twitter link because I tweeted this a while ago, and didn't feel like hosting the image again)
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Duplicate of MC-151.
Tested in 1.4.4 and still works.
Whilst [MC-2372] marks this as fixed.