Does Blowfly Infestation create a loop with Hapatra, Vizier of Poisons?
If there is a creature with 1 toughness on the battlefield when a creature with a -1/-1 counter dies, you can create an arbitrarily large number of enter the battlefield triggers as @gendolkari explains.

This is an optional infinite loop and so the game won't end in a draw because you can always choose to end it by giving Haptatra the a -1/-1 counter from blowfly infestation.  If Haptara has more that 1 toughness, the loop will end because a creature does not die.  If Haptara has 1 toughness, the loop will end because her ability is no longer active.


  421.2. If the loop contains one or more optional actions and one player controls them all, that player chooses a number. The loop is treated as repeating that many times or until another player intervenes, whichever comes first.