My opponent was attacking with a single creature, and targeted it with the instant "Trash the Town", choosing all spree modes targeting that single attacking creature:
- Put two +1/+1 counters on target creature
- Target creature gains trample until end of turn
- Until end of turn, target creature gains "Whenever this creature deals combat damage to a player, draw two cards"

I had Return the Favor in hand:
- Copy target instant spell, sorcery spell, activated ability, or triggered ability. You may choose new targets for the copy.
- Change the target of target spell or ability with a single target.

I had mana to cast the spell with both modes (in the form of untapped treasures in case that matters), and my own creature to target. When the GUI came up allowing me to "Choose one or more additional costs", it allowed me to select the copy mode, but not the change target mode.

While Trash the Town can be cast with multiple targets, in this case as it was on the stack while I was attempting to cast Return the Favor, Trash the Town had just one target. Therefore, my Return the Favor should have been able to change its target in addition to copying it, and the GUI should have given me that option. Even when I deselected the copy option, it still wouldn't allow me to select the change target option.

I did lose the game as a result of this bug.