When attempting to ride an untamed horse under certain conditions (see below), it is unresponsive. It does not walk anywhere (though it does turn on the spot as normal), nor does it attempt to kick the player off, making taming the horse impossible. When not being ridden, the horse acts normally.

The conditions required for this are very complicated and awkward to test, but as far as I can tell, they are as follows;
1. The horse must be on transparent blocks (glass, ice, etc.)
2. The transparent blocks must be at least 4 blocks above ground level.

'Ground level' in this case does not refer to the actual current location of any solid opaque blocks, but rather the height that they originally generated in the world. For example, if a patch of land has generated at y=70, then the horse will be unresponsive if on any transparent block at y=74 or above at that location. This y-coordinate will permanently stay at y=74 even if the player places a solid stone base directly below the transparent blocks, or digs out some of the ground.

This also applies to caves and overhangs; if a cave floor has generated at y=0, the same problem will occur between y=4 and the roof of the cave. This 'ground level' property applies in both normal and superflat worlds.

This bug doesn't seem to reliably happen. In the two hours I spent testing this, there was exactly one case where the horse behaved normally even though it was on an ice platform 5 blocks above the ground. I recreated the exact same setup next to it and the bug reoccurred. I do not know why.