Two friends, Reuven and Shimon, were at a wedding, and the ceremony was about to to start. Reuven cut himself, but didn’t want to leave, so Shimon put his finger on the wound to control the bleeding.
Just then, they announced that Shimon should come up to be one of the two witnesses.
Of course, Reuven came with him.
When the rabbi saw them, he said Shimon was disqualified and they would need a substitute.
When Shimon asked him why, the rabbi replied, “because you’re a banned eid”.