A recent study reports that 1,500 patients leave the operating table with items left inside each year. With roughly 28 million operations each year, that's about 0.005% of all operations in the US.
While that's an astonishing fact, it's interesting to note that the report and article reported the actual number of incidents rather than the percentage that I calculated. It seems to me that this occurs not because hospitals don't care about the patients, but it's cheaper to settle with the occasional "victim" than to x-ray or scan every patient that leaves the operating table. It's all about the Benjamins, baby.