In release 1.2.5 (or strictly 12w17a) and earlier, minecarts would react as one would expect from an entity when walking into one - the player would be met with resistance and the minecart would immediately start to move. However, in release 1.3.1 and later, minecarts appear to have completely lost the ability to push players, and there may also be a third of a second or so delay in them moving in response to intersecting the player, a delay which does not affect any other mobs. They also do not rotate immediately to their new facing direction. This latter flawed behaviour has persisted all the way to the latest snapshot.

This new behaviour is without a doubt linked to the splitting of singleplayer and multiplayer in 12w18a. However, despite the majority of resulting issues having been solved, this and a few others still exist.

Can be easily tested by just placing a minecart on a single rail, breaking the rail, and pushing the minecart around, on both 1.2.5 and 1.3.1 - the physics feel significantly different, and it's possible to walk through minecarts fully unimpeded in the latter.