To my knowledge, this affects all of 1.8 and all of 1.7 along with the newest snapshot.

Minecarts (All kinds) will instantly teleport to a lower rail if it's in the air 1.5 or 2 blocks above it under certain circumstances, through whatever blocks are in the way. If it simply falls into the space above the rail, the minecart will teleport to it at 1.5 blocks above. If the minecart derails directly above another rail, it will instant teleport to the one below the rail it derailed off of. This holds true for a minecart holding an entity.

Minecarts that teleport to the same rail will merge, and if they have the same speed modifier, can be launched out and will move on their own, without rails. Merging can also easily be done with dispensers, but doesn't allow merging of minecarts that already have entities or items in them.