Undopossible=false mana abilites can be undone by invoking 720.

process to reproduce:
1. Have Millikin on the Battlefield
2. Announce a spell that costs more than {1}
3. Tap the Millikin for mana (check your graveyard)
4. Click Cancel to invoke 720 and stop casting the spell.

Expected outcome:
Spell returns to hand, Millikin is still tapped, {C} is in your mana pool

Actual outcome:
Spell returns to hand, Millikin is untapped, card from your graveyard is back on top of your library.

See https://rules.wizards.com/rulebook.aspx?game=Magic&category=Game+Rules#4018
- Players may not reverse actions that moved cards to a library, moved cards from a library to any zone other than the stack, caused a library to be shuffled, or caused cards from a library to be revealed .