[unsafecast]

You say underpromotion, but something people are surprised by is making a second queen ime. People sometimes assume promotion is actually "taking a captured piece back".

[SamPatt]

Very understandable how this developed in the era of over the board play. You can't have two queens when they don't physically exist!

I always use an upside down rook, somehow there's always one available by then.

[unsafecast]

I end up using a checkers piece, since pretty much every board I've played on has them.