When you have an item in your hotbar with an enchanted effect. A written book for example. And you have the F3 menu open, 4 extra pixels appear around the xp bar (see screenshots)
This is not so much of a problem with the default resource pack, with a custom resource pack however this might be a problem. Especially if you are trying to make the xp bar completely transparent.

XP bar without F3 menu:

XP bar with F3 menu:

A custom resource pack to completely remove the xp bar: