That doesn’t seem possible by one person; the max strong-upvote amount at the moment is 9. There’s no downvote-upvote combination that leads to a difference of 13 in the change of 1 vote (though it would be possible for someone to strongly upvote and then change their vote to a strong downvote for a karma drop of 14,16, 18). A more likely explanation is that multiple people downvoted while (multiple people-1) retracted their upvote within the time you were refreshing, or some kind of bug on the forum.
Just saw the karma drop from 50 to 37 in one vote.
This section seems particularly important.
That doesn’t seem possible by one person; the max strong-upvote amount at the moment is 9. There’s no downvote-upvote combination that leads to a difference of 13 in the change of 1 vote (though it would be possible for someone to strongly upvote and then change their vote to a strong downvote for a karma drop of 14,16, 18). A more likely explanation is that multiple people downvoted while (multiple people-1) retracted their upvote within the time you were refreshing, or some kind of bug on the forum.