Fuel for the Cause

• WWizards of the Coast2011-06-01
  Official Ruling
  If the spell is an illegal target when Fuel for the Cause tries to resolve, it won't resolve. You won't proliferate.
• WWizards of the Coast2011-06-01
  Official Ruling
  You must be able to target another spell in order to cast Fuel for the Cause. You can't just cast it in order to proliferate, and it can't target itself.
• WWizards of the Coast2023-02-04
  Official Ruling
  If a player or permanent has more than one kind of counter on it, and you choose for it to get additional counters, it must get one of each kind of counter it already has. You can't have it get just one kind of counter it already has and not the others.
• WWizards of the Coast2023-02-04
  Official Ruling
  An ability that triggers "Whenever you proliferate" triggers even if you chose no permanents or players while doing so.
• WWizards of the Coast2023-02-04
  Official Ruling
  Players can respond to a spell or ability whose effect includes proliferating. Once that spell or ability starts to resolve, however, and its controller chooses which permanents and players will get new counters, it's too late for anyone to respond.
• WWizards of the Coast2023-02-04
  Official Ruling
  You don't have to choose every permanent or player that has a counter, only the ones you want to add another counter to. Since "any number" includes zero, you don't have to choose any permanents at all, and you don't have to choose any players at all.
• WWizards of the Coast2023-02-04
  Official Ruling
  To proliferate, you can choose any permanent that has a counter, including ones controlled by opponents. You can choose any player who has a counter, including opponents. You can't choose cards in any zone other than the battlefield, even if they have counters on them.
• WWizards of the Coast2023-02-04
  Official Ruling
  If a permanent ever has both +1/+1 counters and -1/-1 counters on it at the same time, they're removed in pairs as a state-based action so that the permanent has only one of those kinds of counters on it.

Rulings via Scryfall