Created a 3x3 block wide loop of rails and powered rails and provided 2 minecarts with any mob in both.
They both start looping, but sometimes the minecarts go inside each other and move as one.
They also seem to sometimes just pass over/by me when I am on the track.
They sometimes act as one minecart when being pushed.
Edit: Occurs with at least 4 minecarts with mobs. Also occurred with 1 empty cart and a mob filled one.