This is a form of a feint, or a bluff. This is a very dangerous tactic because if the person making the ultimatum does not follow through, their credibility is undermined and they become a soft target.
This tactic is often phrased as “take it or leave it” or a “final offer.” Many more ultimatums are made than are carried through, especially at the start of the deal. As time goes on, ultimatums tend to be more and more real.
If you make a false ultimatum and should not have, backpedal by framing the ultimatum: “What I meant was…” This is weak, but you already painted yourself into the corner. If this tactic is deployed on you, you must know your walk-away point, and follow through on it if you can’t meet the ultimatum. Alternatively, if you really want the deal but it is more aggressive than you want, try using a conditional acceptance. For example, say: “We have a deal if x, y and z happen in that order.”