The text is a play on the idiom 'A bird in the hand is worth two in the bush,' which means it's better to have a small but certain advantage than the possibility of a greater one that may come to nothing. However, the twist 'a big mistake' does not create a pun as it does not exploit different meanings of a word or similar-sounding words; it simply replaces the latter part of the idiom with an unexpected phrase for comedic effect.
A bird in the hand is a big mistake .