This bug occurs when a piston pushes a rail into the air, and another rail is one block lower and horizontally adjacent.

What I expected to happen was...:
No effect on the existing rail, since pushing the new rail into the air breaks it.

What actually happened was...:
The existing rail bends upwards, as though the pushed rail had remained in midair and they were going to connect with it (even disconnecting from the one below it to do so).

Steps to Reproduce:
1. Build a track of normal rails.
2. One block higher and horizontally adjacent, build a parallel track of powered rails.
3. Place a piston next to the higher track so as to knock a piece of rail off.
4. Place a redstone torch to turn the piston on.