This occurs only with Minecart Experiments enabled.

If a minecart cluster leaves a rail, keeps its velocity due to MC-14, and then reenters a rail, two things can happen:

1. The minecart cluster stops completely. If the blocks are broken below it, it does not fall. It does not respond to any stimulus. (bug3.mp4)
2. The minecart cluster stops. When the block with the rail is broken, the minecart cluster goes backwards. (first part of bug4.mp4)

This seems to depend on the distance the minecart travels without rails (might also have to do with chunk border crossing).

Steps to reproduce:

1. Build one line of rails, then a longer section with no rails, and then again rails (as shown in bug3.mp4).
2. Place a really large amount of minecarts on the first rail track (minecart cluster).
3. Push the cluster so it moves forward.
4. Once it enters the second part of the rail track, the cluster will stop.
5. Break the blocks below it.
6. It will float and remain stationary in space from then on.

If you shorten the section with no rails, the second behavior will occur:

Steps to reproduce:

1. Repeat steps 1 - 3 of the above method, but with a shorter section with no rails.
2. Break the block with the rail on top.
3. The minecart will go backwards until it reaches the first rail track (and now become stationary again).

Note that while reproducing this bug you might have to reload your world if the minecarts disappear (MC-275857).