When riding a minecart into a Nether portal that has only one entrance/exit (i.e. the "back-wall" is blocked), the riding player will get completely stuck. Neither leaving nor breaking nor moving it will work. The player will not be able to continue playing in SSP normally in this situation.
On SMP the player needs another player to push him/her out of the portal to continue playing.
With players' inability to ride through Nether portals unfortunately being intended behavior (according to Dinnerbone's comment on
MC-1969) this is a somewhat serious bug.
- 2013-03-21_23.34.49.png: The test track. Riding it will result in the player getting stuck.
- 2013-03-21_23.35.46.png: A view from a stuck player. Neither moving, breaking or leaving the minecart will work.
- 2013-03-21_23.35.54.png: Trying to break the minecart will make particles appear as if you're trying to break a portal block.