Minecarts that run next to each other will occasionally push each other, especially on corners. This means that automatic farms with hopper minecart collection systems can be broken when the minecarts stop each other when they pass each other on a corner, or sometimes even on a straight section of track.

Steps to replicate:
1. Make a square out of rails, then another square on the inside of that so that both sets of rails are touching each other around the whole square. Put plenty of booster rails along the tracks to keep the minecarts running.
2. Run minecarts along both rails in opposite directions.
3. Just wait for the minecarts to stop or turn around when they pass each other.

UPDATE:
I have recently found this bug to happen on lone minecarts running by themselves.