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.
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Even around 1.7, the visuals for the hitboxes changed, but the player hitbox was still centered too high.
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The bug was eventually fixed in snapshot 14w06b. Excuse the Steve skin in this screenshot.
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Relevant Minecraft bug report: https://bugs.mojang.com/browse/MC-4077
The one with the funny mouse logo. (XFce)