Adding Irrelevant Stamps On A Cover

In the US you can add as many stamps as you like so long as you have at least the minimum amount of postage required for that letter or package. People who use older postage stamps these days (stamps from before the ones that are "forever" stamps) often overpay slightly in postage since it is hard to find an exact combination of values to add up to the correct postage amount. And I have seen silly things like creative collectors who take older first day covers and add modern stamps that relate to the original first day cover stamp or theme and have them hand cancelled. For example, adding one of the new Florida Panther forever stamps to a FDC for the old 26 cent Florida Panther stamps to create a kind of "double panther first day cover"