Doing a Mace

Announcing that you intend to do something but then pull out at the last minute much to the annoyance of everyone involved.

Person 1: Hey, would you like to move in with me?
Person 2: Of course.

Day before moving in-
Person 2: Oh I can't move in, i'm a complete bastard.
Person 1: 'Doing a Mace' I see, no more being alive for you!
(Person 2 is beating to death with a hammer as Person 1 is very annoyed)

Doing a Mace

To pull out of a commitment at the latest possible opportunity, often to the severe detriment of other parties involved.

Person 1: "Dude, can I have a kidney?"
Person 2: "Sure"

On operating table:
Person 2:"Lol jk, no kidney for you"
Person 1: "You're Doing a Mace! Noooo"*Dies*