When a powered rail facing downwards is placed next to a solid block, a minecart is able to pass through it if the minecart is pushed forward into the block by a powered rail. I've included a GIF of the minecart going through the block below.

 Steps to Reproduce

1. Place powered rail at a downward angle going into a block so that the powered rails path is blocked.
2. Power said rail with redstone, the minecart will phase through the block that is blocking its path

GIF of bug in action:

Link to Imgur GIF