I just encountered a version of the -1 review bug: -1 +1

How it happened: I added a few recently-learned grammar items to my reviews, and in the middle of doing them I encountered an item that I thought I must’ve added by mistake since it wasn’t one of the grammar points I’d just been reading up on. I removed it from the reviews without answering it, and then belatedly realized that it had been one of my regular reviews. Afterwards, I found the grammar point and re-added it to the reviews – but despite the “NEW +1” text, it didn’t put it back in the review queue immediately (looking in upcoming grammar, it’ll be in 2 days).
The “-1 +1” was persistent, even if I went to another page. Clicking on the reviews button did nothing, as there were zero available at the time. Once some reviews in my queue became available, the count shown was 1 less than it was supposed to be, but I was able to get the count back to normal by using the “finish” option in the middle of the review session. I didn’t test whether completing the reviews normally would fix it.