The fact I was allowed to close it.
If you just load a pinned tab normally, and just hit Cmd-W to close the tab, it doesn't actually disappear as a pinned tab. However in the overview, if I click that X, I lose my pinned tab.
It should either just not have an 'x' close option, or it should have it, but the effect should just be to unload it, not to actually delete the pinned tab
Pinned tabs are pinned...they should survive typical acts of closure.