To be in the heat of the moment means to be overwhelmed by a situation.
1. The work pressure was so high that in the heat of the moment Meenal mistakenly mixed up the documents.
2. Taking part at the Olympics for the first time, in the heat of the moment the pole vaulter fouled up her run-up.
3. In the heat of the moment the coach snapped at the reserve player for day-dreaming instead of preparing to go on the field, as precious seconds were being wasted.