Blightbeetle + Viscera Seer + Switcheroo

Blightbeetle

Viscera Seer

Switcheroo

How it works

1. Donate Blightbeetle to an opponent
2. Activate Viscera Seer by sacrificing the undying creature.
3. When the undying creature dies, its undying ability triggers, returning itself from your graveyard to the battlefield without a +1/+1 counter on it due to Blightbeetle.
4. Resolve the Viscera Seer ability.
5. Repeat from step 2.

This fascinating loop relies on giving your opponent a Blightbeetle, whose static ability completely shuts down the placement of +1/+1 counters on creatures they control. Because of this restriction, any creature with undying that you control (though missing from the direct list, implied by the steps) will continuously cheat its return condition, coming back to the battlefield with no counter to stop it. You can then use Viscera Seer to sacrifice that creature repeatedly, feeding its sacrifice outlet to endlessly trigger its death and rebirth while digging through your deck with infinite scry triggers. Finally, Switcheroo makes this all possible in the first place by letting you hand the Blightbeetle over to your opponent's side of the table so its counter-hating aura can do its work.