The Tigers were flying high after three successive Championship wins, but followed up a 3-1 defeat at Leicester with a home defeat by the same scoreline at the weekend to bottom club Peterborough.
"The players know," Bruce said.
"The first goal wasn't a defender's fault. It was (midfielder Stephen) Quinny, who made the wrong decision. One thing you can't do as a manager is stop individual errors."
"Complacency comes in, we've told them to avoid it. In this divison, in my experience, the ones who make the least mistakes come out on top."
"A week ago we were beating Leeds. We have to show what we're made of against Blackpool (at the KC Stadium on Tuesday night) and start again."