Cathars' Crusade, Nest of Scarabs, and Barrenton Medic

Prerequisites

• Cathars' Crusade on the battlefield
• Nest of Scarabs on the battlefield
• Barrenton Medic on the battlefield
• Barrenton Medic does not have summoning sickness
• Barrenton Medic does not have more than two -1/-1 counters on it

Steps

1. Activate Barrenton Medic's first ability by tapping it, preventing the next one damage done to it this turn
2. Activate Barrenton Medic's second ability by putting a -1/-1 counter on it
3. Nest of Scarabs triggers, creating a 1/1 Insect creature token
4. The Insect enters the battlefield, triggering Cathars' Crusade, putting a +1/+1 counter on each creature you control
5. Resolve the Barrenton Medic ability from step 2, untapping it
6. Repeat

Results

• Infinite creature tokens
• Infinite ETB
• Infinite +1/+1 counters