when you are ingame and have a unit that is capable of being upgraded twice (e.g. Angler / Cursed Casket) and you do both upgrades within one building phase, then you can just "revert" the last upgrade for 100% cashback. It doesn't work for both of the upgrades.
#Refunding of a 2-Upgraded unit is not possible
patent river
devout hull
its a long time known bug