I've found this bug first appear in the 14w25a snapshot and it is still around. It's a graphical glitch for the crosshair of custom resource packs. It's not appearing in the default resource pack.

The way this bug is triggered is very strange and I will attempt to describe it as thoroughly as possible. I should first mention that my custom crosshair is just a single pixel in the middle.

To trigger this bug:
Hold any stack-able item in your hot bar. The stack-able item has to be in a singular form and has to be of the stack-able variety. The item has to be on the far right of any other hotbar items. If done correctly, it will smudge the crosshair to a shape similar to the default.
I'll try to explain some instances and use 1-0 to represent the slots. The bug is not triggered specifically with the items listed in the example.

1. If you have a singular apple and you place it on slot 8, have 9-0 empty, and have any variety of items in the other slots, the bug will trigger.
2. If you have that apple in slot 5, have some items in slot 6, and slots 8-0 empty, the bug will not trigger.

It seems to be completely dependent on whether a singular item is the last item in the hot bar.
Here are some screenshots and notice the location of the test item, the singular apple.