What I expected to happen was...:
i was trying to push a detonated tnt block in the air. and then just went on experimenting and added another piston.

What actually happened was...:
the detonated tnt got pushed against a redstone torch and the redstonetorch changed into a tnt block itself, the tnt block that was created changed back to a redstonetorch as soon as it got triggered by redstone or by a placed block next to it.
If you relog while the blocks have been changed to tnt etc. it will be as like it never happend.
https://www.youtube.com/watch?v=qyr1d7r7nas
https://www.youtube.com/watch?v=USjETJhKGA4
(sorry for the darkness of the videos)
Steps to Reproduce:
1. create the stuff seen on the video above. (piston with pressureplate next to it, tnt on top of the piston, another piston thats 1 diagonal from the tnt and make the pressureplate connect to the piston then make a block (not bedrock or obsidian) with a redstonetorch on it 1 the other diagonal side from the tnt. The water is just to prevent the tnt from blowing, doesn't change the bug.)
2. If you are in multiplayer the 2nd piston needs to push slower and you have to place the tnt on the piston as soon as you step on the pressureplate.