Hmm. What if this replaced other players' life gain? It would be more black to me that way.

Making it a replacement effect causes an infinite loop if more than one player has one. Note that this version has a "safety valve" in case that situation occurs.

Of course, this card is only black because the original version was a replacement effect, so maybe I should just make it white instead.

Just a note: replacement effects can never cause an infinite loop, because each replacement effect can only apply once to a given effect.

If this were a replacement effect - "If another player would gain life, instead you gain twice that much life" - and both players had one, then the net result of me casting Sacred Nectar is me gaining 16 life. Just like two players having a Notion Thief doesn't go infinite, nor would this.

Oh, huh. The more you know. Thanks Alex.

Replacement effects are capable of causing a game draw for an endless loop; see Comeuppance vs. Comeuppance. That case is a bit unusual, though, since each prevention effect is also creating a brand-new damage event that the other hasn't applied on.

Ha! Yes, okay, trust SadisticMystic to find the extremely rare exception to my general statement :)