I was currently at the Overworld at the time, and I was underground working on my adventure map. While doing so, I spawned a sheep because I felt lonely (I don't think this has anything to do with the bug, but it's nice to know incase it is part of the bug). The current coordinates I was at the time were: -484, 42, 111. When I went above the mysterious border, it disallowed me from going through it, acting a bit like a slime block, but invisible, although the actual border size was only about a slab big. I tried walking on it and flying under it and hitting it from underneath but I kept bouncing. I was using a potion of Night Vision for 8 minutes at the time, too.

I managed to fix it by Saving and Quiting, quiting minecraft itself, and rejoining the game.

And it's happened again, but in a slightly different area. I even got a sheep this time to demonstrate what it is doing. In the photo with the black sheep, what I am trying t show is the sheep hits the dirt below, but randomly flies up to where the border was, and repeats this.