I was trying to build a gold mine in the mesa, I encountered a bug while testing the minecart. When the minecart move up a one block ramp, the minecart would glitch really badly, and glitch in the block under the rails. This only happens when the carts go upwards. Sometimes the minecart would completely disappear and no item would be dropped.  Here's some screenshots. NOTE: I'm using a ported new minecraft defaults texture from Java. 

I really hope the developers can resolve this because, when 1.7 is released this glitch will break a lot of people's farms!