Searched, couldn't find this. I do recall seeing a YouTube video on an issue identical to or similar to this, but can't find it either to see if this duplicates anything.

It is possible for a player to visually appear standing up in a position that cannot be accessed via walking in a standing up position, such as a top-half closed trapdoor above a repeater.

Attempting to walk into this arrangement will fail. However, since sneaking changes the player's model to be slightly shorter, the player can sneak into this gap. When inside it, the player can stop using the sneak key, but the player's vision height will not change when in first person view. When in third person view, the player will appear to be standing up, which makes no sense as it should not be possible to stand up in this space. The player should still be adopting a sneaking pose instead, even if not truly sneaking.