What I expected to happen was: I press left-shift to exit a minecart and I end up standing next to it.

What actually happened was that I dropped through the ground and fell until I hit something else. I have train station platforms in the Nether that are suspended in the air and are one block thick but have a complete tunnel with roof and sides and at least one row of blocks next to the tracks at all points. The spots I fell through (it's reproducible and appears to happen every time) are made of netherack. Any exiting along this suspended 1-block thick track platform causes me to go through it. At one station I go through the platform, then hit air, then hit netherack maybe 20 spaces down. At the other station I fall maybe 40 spaces down into a lava lake. The player is using a skin file and was wearing armor. The game is not modded.

Steps to Reproduce: No way to test all combinations on my own, but what seems to reproduce it is to get into a minecart on a track that only has one layer of blocks holding it up and then air below and then exit--I consistently go through the platform blocks instead of standing on them.