Double-ace flush

Some home poker games and some casinos play the double-ace flush rule, in which a wild card in a flush always plays as an ace, even if one is already present. In such a game, the hand A-Wild-9–5–2 (of one suit) would play as if it were A-A-9–5–2, defeating A-K-Q-10–8 (of one suit), whereas by the standard rules it would play as A-K-9–5–2 and therefore lose (the opposing hand's queen outranks the first's nine). This rule is rare, and is an exception to standard practice, so it should be announced clearly if you intend to use it.