In Magic: the Gathering and other popular card games, advanced players have developed the notion of a "win-more" card. A "win-more" card is one that works very well, but only if you're already winning. In other words, it never helps turn a loss into a win, but it is very good at turning a win into a blowout. This type of card seems strong at first, but since these games usually do not use margin of victory scoring in tournaments, they end up being a trap-- instead of using cards that convert wins into blowouts, you want to use cards that convert losses into wins.
This concept is useful and important and you should never tell a new player about it, because it tends to make them worse at the game. Without a more experienced player's understanding of core concepts, it's easy to make mistakes and label cards that are actually good as being win-more.
This is an especially dangerous mistake to make because it's relatively uncommon for an outright bad card to seem like a win-more card; win-more cards are almost always cards that look really good at first. That means that if you end up being too wary of win-more cards, you're going to end up misclassifying good cards as bad, and that's an extremely dangerous mistake to make. Misclassifying bad cards as good is relatively easy to deal with, because you'll use them and see that they aren't good; misclassifying good cards as bad is much more dangerous, because you won't play them and therefore won't get the evidence you need to update your position.
I call this the "win-more problem." Concepts that suffer from the win-more problem are those that-- while certainly useful to an advanced user-- are misleading or net harmful to a less skillful person. Further, they are wrong or harmful in ways that are difficult to detect, because they screen off feedback loops that would otherwise allow someone to realize the mistake.