Minecraft 1.6.1, Minecraft 1.6.2
If you place a powered rail on a block with a powered rail connecting to the block with the powered rail, if the minecart is on the rail that is sloping up to the block you can't see anything out of the minecart if your in the minecart. Normal minecarts without a slope are fine.
X: Block C: Minecart Y: Powered Rail
YC <------- If your in the minecart at C you can't see anything.