"Sleeper success" refers to something, like a movie, TV show, or book, that initially goes unnoticed or does not receive much attention, but later becomes very popular or successful. Full definition
Last night word spread round that this summer's sleeper success Wind River, about a rape / murder investigation on an reservation, had possibly found a new lease on life. (thefilmexperience.net)
But some supremely effective chills and good word of mouth could spell sleeper success for this Aug. 7 opener. (variety.com)
After the enormous sleeper success of 1999's The Sixth Sense, M. Night Shyamalan suffered a modest setback with his next flick, 2000's Unbreakable. (dvdmg.com)