I have only witnessed this bug once, while playing in a realm I host for myself and a few of my friends. In a world with 2 nether portals in the overworld and in two in the nether, one linked to each for quicker travel, and after sending my horse through one of the portals, I stepped through, but when I was transported to the overworld, my coordinates had not properly transformed to overworld coordinates. Specifically, my coordinates while in the nether were 69, 84, and between -98 and -99, as the portal was two blocks wide, and my overworld coordinates should have been between 552 and 553 for the same reason, 106, -782, but I was instead placed at 63, 80, -98, by my best guess, as I did not record the precise coordinates, but remember a rough idea of where I spawned, and, since both 69, 84, -99 and 69, 84, -98 would have placed me inside of terrain, I can assume some sort of fail -safe was triggered which placed me outside of the terrain and on the ground so that I did not die of fall damage. From what I can recall, though, the terrain which surrounds my best guess at the coordinates which I was actually placed at in the overworld do not quite match up with the terrain I remember, so those may not be the actual coordinates, but from what I can remember, the coordinates were close enough to the ones at the nether portal in the nether. I mention the horse because that is the only significant difference between normal nether transport that I can think which may have triggered the bug. Three others were in the world at the time, though they were near the spawn chunks at about 700, y, 0 in the overworld. Also, I would like to note that I have traveled with my horse other times through the nether and have not experienced this bug more than once. No nether portal was generated near 69, 84, -98, but the swirling of the screen and the purple tint consistent with stepping out of a portal were present, for only a moment, naturally fading away as if I stepped out of a portal. The seed for the world is 253010118, generated in 1.8.0 or 1.8.1, if the latter exists.