Scenario:
Two pistons are facing each other with two blocks of space separating them. A solid block is placed between them. Placing a repeater against the solid block and powering the repeater powers the block behind the solid block (from the perspective of the repeater). Place redstone to transfer that power to both pistons, so they will both start pushing the block -> It disappears.

Don't ask me what I expected to happen, I'm a noob that was simply experimenting. However, I certainly wasn't expecting this . I'm not sure what would constitute correct behavior for this case either...

Link to world (go to 0,50,0): https://www.dropbox.com/sh/lbyw0ww5ght5e16/6l0k15UUva/Minecraft%20Bugreports