Most firms evaluate the round using the combined evidence from the interviews rather than requiring every interview to be equally strong. A weaker interview does not automatically erase a strong one.

The concern is whether the weaker case revealed a fundamental issue that also appeared elsewhere, or whether it was one contained performance dip.

Do not try to reverse-engineer the outcome from your own feeling alone. Interviewers may score parts of the case differently from how candidates perceive them. If useful, this guide on first-round consulting interviews gives more detail.

One weak one doesn't always kill the round.

I had one great / one meh and still moved on.