The turn prior, Alice played the infinite life combo and went to "infinite" life. By the rules she can't actually go to infinite life, just some very high but finite number (let's say 1 Googol). Then she passed the turn. Bob played Walking Ballista for 2, activates Heliod, and starts shooting. What happens now?
- Bob: shoot you for one? (Alice says OK) Gain life, pump my Ballista, shoot you for one? (Alice says OK) Loop 1 Googol times?
- Alice: OK, after your 1 Googol activations, with the 1 Googolth activation on the stack, I activate my Spike Feeder and gain 2 life?
- Bob: Okay, after you gain two life, shoot you 1 Googol times?
- Alice: In response, gain 1 Googol life?
... and repeat.
Looks related: What happens if both players have the ability to pump their creature an unlimited number of times? However the two infinite combos above might never lead to the same state, e.g. Bob can say "I shoot you half a Googol times", and it's not clear what life total Alice has afterwards (if she survives).