Vault cap - it’s most annoying during festivals. It’s practically impossible to get that “just about” amount of raid gold to not go over the limit too much as it is, let alone with bigger loot from festival raids.
What’d help is if the vault capacity limit was removed during festivals. I’ve suggested before that you could have more gold than you can hold until you log off, this could work similarly. You have, say, 13.6 million gold, you win a raid for 2.5 million, so you end up with around 16.1 million. You immediately spend 15 million on an upgrade (btw who’s idea was it to require 100% gold for an upgrade, and has he been slapped with a wet cabbage for his brilliant idea? ) leaving you with 1.1 million in the vault that would’ve been otherwise wasted. Isn’t that a lot better? It’s A LOT better for festivals when you try to maximise your income for most efficient upgrades.