Premoved Castling - King Ended up Moving Twice in a row without castling

Really odd bug that happened in this game. I premoved castling, and it seems like the server took it as I premoved the king twice in a row to move to the left.

Here's the link to the game, and it happens on move 16: lichess.org/XGHWF3uPZJUm

I still ended up winning the game after asking the guy to do a takeback, but it was really the oddest thing I've seen while playing online chess.

Sorry for the dumb question, but how do I duplicate this? (What browser version?)

you most certainly just premoved Kf8

I suggest getting better internet. It took 3.6 seconds for the premove on move 16 to go through, at which point the king probably got lazy and decided to move one square instead of two. It's also possible that the bishops advised the king to move only one square, so you could exchange the bishops to decrease the chance of this happening.