a/one false move

false move

 and one false move
Fig. [even] a single movement that indicates that one is disobeying an order to remain still or in a nonthreatening posture. The robber threatened to shoot us if we made one false move.
See also: false, move

(make) a/one false ˈmove

(informal) in an already dangerous or risky situation, something which makes your position even more dangerous: She’s in a difficult financial situation, and if she makes a false move now she could lose everything.‘One false move and you’re dead,’ he shouted at the bank clerk.
See also: false, move, one