A mysterious class of entity which exists as a sub-class of every class for which pundits pick wins. ``Sleeper hit'' is the sub-class of entities which the pundits weren't paid/bribed enough to promote much but which the consuming public liked.
Sleeper hits are common in the film and book industries, but rely on access to distribution for content made outside the big companies. Thus any monopoly of cinemas, bookstores or other media distribution points is likely to suppress sleeper hits and lower the general quality of entertainment media.
There are three ways to determine whether an entity was a sleeper hit. (1) rather than opening in the record/film/book charts, it took several weeks/months to climb into the charts. (2) the first time reviewers refer to it is after the opening and the review is prepended with the word "surprisingly" and (3) you hear about i from your friends before you hear about it on TV