We were just talking about Dave and his plans for the holidays when he walked through the door. Speak of the devil… (Wenn man vom Teufel spricht…)
While nowadays this phrase is a light-hearted way of referring to someone who appears as they are being talked about, before the 20th century it wasn’t light-hearted at all. It comes from the old suspicious belief that it was dangerous to mention the devil by name. Sound familiar to you Harry Potter fans…?