(This seems like something someone would have noted already, but a cursory search here did not find any such report.)
I had a tab open with the close review queue for a specific question. For whatever reason, I ended up opening the question itself in a new tab. I then voted to close the question from the new tab, not from the review queue. Then I went back to the tab with the review queue and I was able to vote to leave the question open.
In other words, I was able to vote to leave open a question after already voting to close it.
This seems like something that I shouldn't be able to do (at least without retracting my close vote). Indeed, if I refresh the tab with the review queue after I vote to close from a different tab, I cannot vote to leave open. Instead I get a message that the question is no longer reviewable. It also remains unreviewable even if I subsequently retract my close vote and refresh the tab again.