With the introduction of Minecraft Java 1.4.3, they added the F3+B functionality to view all the hitboxes on entities. However, it wasn't perfect. The player hitbox renders too high. This can pose a problem for my main instance as well, as it is within this version range.
Even around 1.7, the visuals for the hitboxes changed, but the player hitbox was still centered too high.
The bug was eventually fixed in snapshot 14w06b. Excuse the Steve skin in this screenshot.
Relevant Minecraft bug report: https://bugs.mojang.com/browse/MC-4077
