Minecraft wiki entry for Hitbox, associated text: The arm of an extended piston - specifically the part which protrudes into the piston's block space.[1]
The hitboxes of pointed dripstone in Java Edition 20w48a could sometimes also randomly extend into adjacent blocks, allowing this to be seen.[2]
The hitboxes of melon stems and pumpkin stems with ages 8[verify] and over could visually extend into the block space above them while they existed (from Beta 1.8 Pre-release up to the removal of the last four in 14w10a and the remaining four in 14w25a).
The hitboxes of cakes with 8[verify] or more bites could extend into the adjacent block space while these existed (from Beta 1.2 up to their removal in 14w26a, although 14w25a likely made them too unstable to be properly observable)
The hitboxes of cakes and cacti at far distances could previously (from Beta 1.2 and Alpha v1.0.11 respectively up to their fixing, likely simultaneously in 15w49a, although it became unobservable for cake from 15w38a onwards due to MC-106300) also become greater than a full block, allowing this to be seen.