I also discovered this problem on a multiplayer server.
For me it happend also in the nether. The tracks are inside a tunnel, 1 block free between the tracks and the walls. When i exit the minecart when the tunnel is above a lava see, I fall into that sea. When i exit when the tunnel is on solid blocks, I find myself standing outside the tunnel walls. So my suggestion of what happens is that the game can't place you directly besides the tracks and so it takes the next free space beside the track which is outside of the tunnel.
Heiko Seifert
I also discovered this problem on a multiplayer server.
For me it happend also in the nether. The tracks are inside a tunnel, 1 block free between the tracks and the walls.
When i exit the minecart when the tunnel is above a lava see, I fall into that sea. When i exit when the tunnel is on solid blocks, I find myself standing outside the tunnel walls.
So my suggestion of what happens is that the game can't place you directly besides the tracks and so it takes the next free space beside the track which is outside of the tunnel.