
"Fool me once..." is an alright card if played fairly, for it purpose of cancelling a future encounter card - it comes cheaper than Ward of Protection (since you don't have to pay a horror), but costs an extra XP and has more stringent restrictions on when it can cancel a card. Other reviews give more in-depth descriptions of this.
However, there's an important (perhaps unintended) usage of this card: unlike most treachery-interaction cards, this one doesn't specify that it only applies to non-weakness treacheries, so you can use it to trap a weakness to avoid redrawing it when you reshuffle your deck.
Most Guardians don't really have the draw to take advantage of this, but this effect can be especially strong in Joe Diamond, who does have access to the tools needed to repeatedly draw his deck, and this can let you really easily facilitate infinite combos, even if you draw a particularly nasty weakness (such as Amnesia).