Likely a small issue present when Notch coded it (related to MC-5024 in that respect, and both are texture positioning errors).

The hotbar selector (image to designate currently held item in hotbar) is improperly mapped, only using a 24x22 px selection rather than the 24x24 px square it should use.

One row of pixels isn't visible because it isn't mapped that far. Another isn't visible because the hotbar touches the bottom of the screen, meaning the selector (which is taller than the hotbar) would go off of the screen.

These errors aren't visible in default because the selector and hotbar both have a black outline. Attached is a debug pack and an image of what it looks like with that pack. The pack includes a selector image that has different square outlines so the issue is apparent.