It seems when too many minecarts are near to each other, they will fusion into one big minecart mess and sometimes stop moving along the track, simply spazzing in a restricted area.

This bug is reproduced by:
1. Create small crircular track
2. Send several minecarts into said track.
3. Keep adding minecarts at a slow rate, until they become one or two static minecarts, slightly twitching along a vertical axis, but not moving along the rails.

As this event is difficult to describe, i have included a picture below of before and after the bug. (Before is the one with more minecarts)