Complication
This consequence represents trouble, mounting danger, or a new threat. The GM might introduce an immediate problem that results from the action right now: the room catches fire, you’re disarmed, the party takes +1 trouble from evidence or witnesses, you lose status with a faction, the target evades you and now it’s a chase, reinforcements arrive, etc.
Or the GM might tick a clock for the complication, instead. Maybe there’s a clock for the alert level of the guards at the manor. Or maybe the GM creates a new clock for the suspicion of the noble guests at the masquerade party and ticks it. Fill one tick on a clock for a minor complication or two ticks for a standard complication.
A serious complication is more severe: reinforcements surround and trap you, the room catches fire and falling ceiling beams block the door, your weapon is broken, the party suffers +2 trouble, your target escapes out of sight, etc. Fill three ticks on a clock for a serious complication.
Don’t inflict a complication that negates a successful roll. If a PC tries to corner an enemy and gets a 4/5, don’t say that the enemy escapes. The player’s roll succeeded, so the enemy is cornered… maybe the PC has to wrestle them into position and during the scuffle the enemy grabs their gun.