In monobook the text inside the tab is slightly too high, which is overlapping with the top border of the tab to some degree. This is because it's an OOUI ButtonElement that's been stuck inside a tab, and there's a lot of styles that don't expect this.
See e.g. https://en.wikipedia.org/wiki/Template:Sandbox?useskin=monobook&safemode=1
Ahchet's screenshot:
I see it less severely:

