Alrighty with the image it should be pretty clear whats the glitch and how it occurs but if your confused I mine aswell make a description anyways.

When in F3, and looking at a non full block around your camrea will appear a black box, obviously this should not be in the game so I expect you to fix it like you people have been doing for the past 11 years (thanks for that) anyway another way the thing can occur still takes place in F3 but your characher may be "inside" a block like next to a honey block or a cobblestone wall, which also causes this "effect".+