Being forced into 1.5 sneaking while in 3rd person mode causes a flash.

The flash is caused because of the 3rd person camera being forced into a 1.5 gap, which causes it to flash into the skin before entering the 1.5 3rd person camera mode.

To recreate the bug, place a trap door at the ground and one about 2 blocks high. Then open the one on the top, go there, go into 3rd person camera, close the one at the top.

And here is a video.