MTG Combo: Glen Elendra Archmage + Phyrexian Altar + Spark Double

This infinite combo generates infinite mana, and creates an infinite sac target. All it needs is a free sac outlet (in this case Phyrexian Altar) and any persist creature. All you need is that Spark Double to enter the battlefield copying the persist creature, then sacrifice it to the sac outlet. Persist triggers and it enters the battlefield again with a -1/-1 counter, choosing the persist creature to copy again, so it also enters with a +1/+1 counter, negating the -1/-1 counter from persist, letting you sac it again. Repeat for infinite sac triggers.