A perfect example of when to tell something, in the old adage of showing vs. telling, is from Anne Rice's Interview with the Vampire. In fact, it's in the opening lines:
"I see . . ." said the vampire thoughtfully, and slowly he walked across the room towards the window.
The "tell" is that the subject is a vampire. Anne didn't say it was a man with white, smooth skin, almost like scupted stone. She does this on the second page, but her opening lines tell us that this character is indeed a vampire. We're not guessing here. We're jumping right into who the main character is, no questions asked.
Most of the time it's good to show instead of tell, but in this case it serves a far greater purpose.