When selecting part/s of 1 or more objects, or duplicating with the gumball, the option to retain that custom selection AFTER you have undone the action (ctrl + Z). Often, I’ve spent heaps of time making a custom selection, performed an action which wasn’t right, had to undo, and lost the custom selection. Very time consuming.
This also occurs when duplicating an object with alt on a gumball arrow and undoing the action
Please see the video explanation, as it’s easier to communicate the issues.