An older question popped into the review queue on my beta site today. It had a whole lot of close votes but most of them had expired, so the question never actually got closed. Back when it was first posted, I'd left a comment on it explaining to the OP why it was off topic (as had several others). At the time it was asked I did not have close voting powers, or shiny moderator ones.
So anyway, for whatever reason the question popped back into the review queue today. It clearly needed to be closed, so I went to do so. I wanted to close it as Off Topic -> Other, because none of our specific close reasons fit. I didn't want to leave another comment though; there really wasn't any point, I'd said everything that needed to be said previously. So I removed all the text from the "custom reason" button, and tried to submit the closure. But it wouldn't let me, claiming I had to enter a comment.
Well, I already did enter a comment! Just not at the same time as I tried to close it. The comment is already posted on the question and explains the reason why the post should be closed. There's no need to add another comment, that's just clogging up the OP's notifications for an old question (and being redundant). I suppose I could have left another comment and deleted it right after, but still.
I think that the closure system should check and see if you have already left a comment on the question. If you have, it shouldn't require you to leave another.