18. Dick and Harry are discussing Tom while he isn't around. Suddenly, Harry mutters "speak of the devil."
What has just happened?
From Quiz Some Devilish Idioms
Answer:
Tom has just unexpectedly appeared in their midst
This idiom is believed to have originated in the Middle Ages, where it was considered bad luck to speak about the devil, or evil in general, as it was considered to be an invitation to the devil to appear. The idiom is basically a shortened version of the original English saying, "speak of the devil and he shall appear" or "talk of the devil, and he's presently at your elbow." It's an idiomatic instruction that warns against talking about people behind their backs, and tempting fate.
Funnily enough, most languages have their own version of this idiom, with many of them referring to a wolf rather than the devil. One that I found particularly amusing is the Swedish "när man talar om trollen så står de i farstun", which translates to "when you speak of trolls, they stand in the entrance hall."