I found it odd that when I unioned something successfully, undid the operation, and attempted the same operation again, the operation which had just worked previously would fail. Selection order matters when unioning a selection of parts, and this was the reason. Since selection order is not preserved in undo history, when the union is undone it ends up in a bad selection order that doesn’t work anymore.
Video:
I had to use a complicated union for demonstration because simple unions always are successful regardless of selection order.
Would you considering adding selection order to the undo history so this wouldn’t be an issue? Even in the future it didn’t error because CSG was perfected, bounding boxes are still determined by selection order, so it’s essential for the selection order to be preserved when undoing a part operation in order to create the same union again without re-selecting them in that order by hand.