Ordinarily, sneaking prevents the player from walking over gaps. However, if wearing leggings enchanted with Swift Sneak III, the player can sneak over an open trapdoor while moving diagonally. I have seen this happen when walking directly forward, although it is very inconsistent. (Diagonal movement, like showcased in the attached video, happens every time.)

UPDATE: I noticed it happening more on my realm, and have discovered that this can happen with larger gaps while having more speed. This also applies when the player's speed is increased in any way. For example, the same behavior occurs with Speed VIII, using the same gap size. With Speed XV, it can be done while moving directly forward. Moving over a gap directly forward can also be achieved easily by combining Swift Sneak III with Speed II. This behavior can even be observed over small gaps without increasing the player's speed. The player can sneak from a wall to a fence at the player's base speed, despite being able to walk between them. Sneaking between a wall and a fence is not possible with Slowness III, and shows the same unusual behaviors with Slowness I & II.