Shimon's field was stolen by Reuven. Yehudah now claims the field is his. Can Shimon testify on Reuven's behalf that the field isn't Yehudah's?
Shimon has a stake — he wants the field to stay with Reuven so he can sue Reuven and recover it. His testimony would be biased, and he is disqualified as a nogea b'eduto.
Question 2
What changes when the thief (Reuven) dies before the dispute is resolved?
Once Reuven is dead and the purchaser acquired the item through yiush and change of domain, recovery is impossible. Shimon now has no interest in who keeps the item — so he may testify.
Question 3
A seller sold a garment to Shimon without warranty. Yehudah claims the garment. Can the seller testify for Shimon?
For movable property, a seller's creditor generally cannot expropriate sold items from the buyer. So the seller typically has no financial stake, and may testify — provided the buyer acknowledges the seller's original ownership.
Question 4
Which individuals are acceptable as witnesses but not as judges in capital cases?
Rambam lists: friends, enemies, converts, freed slaves, elderly men, eunuchs, bastards, and those blind in one eye as acceptable witnesses but not judges in capital cases.
Question 5
Why can't two related judges sit together on the Sanhedrin?
Rambam explains that the disqualifications that apply to witnesses apply equally to judges. Since relatives cannot testify together, they cannot judge together. The same conflict-of-interest logic governs both roles.