How does a -1 vote happen?
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I'd asume someone downvoted, then removed the downvote. The original downvote didn't get through, but the reversal did. Something like that
I do that constantly by misclicking.
If you use Voyager or some other front end that uses gestures on a touchscreen, you can sometimes accidentally up or downvote when scrolling or navigating.
my instance blocks downvotes
That’s my guess too.