when Rav Hamnuna once happened to come to a place called Darumata he heard the sound of a shofar announcing that a person had died in the town. When he saw some people doing work he said to them: Let these people be under an excommunication. Is there not a dead person in town? They said to him: There are separate groups in the town, each one responsible for its own dead. Knowing that the deceased was not from our group, we continued our work. He said to them: If so, it is permitted to you, and he revoked his excommunication.
How come Rav Hamnuna pronounced a sentence of excommunication before hearing out all the circumstances?
Cf. Yevamot 121a where Rav wants to excommunicate Rav Sheila immediately, but Shmuel demands an investigation.