Sein blaues Wunder erleben

Literally: To experience your blue wonder

Example: “Wer diesen Vertrag nicht liest, erlebt sein blaues Wunder.” — Anyone who doesn't read this contract is in for a rude awakening.

English equivalent: To be in for a rude awakening

Origin: Blue (blau) once stood in German for deception and lies, as in jemandem blauen Dunst vormachen (to blow blue smoke at someone) and das Blaue vom Himmel herunterlügen (to lie the blue out of the sky). A blaues Wunder is a marvel that turns out to be a nasty trick, and the phrase is recorded from the 15th or early 16th century. The lexicographer Küpper pictured old conjurors clouding their audience's eyes with blue vapors; another explanation points to dyers whose cloth turned an unexpected color as it dried. None of these is certain.