Revealing mistake: Almost every time Sam looks in a mirror or the viewer sees Sam in a mirror, the movements of Sam and Sam's "reflection" hardly ever match up correctly as if it were Sam's real reflection.
Revealing mistake: Al is supposed to be a hologram. However, he is constantly casting shadows, and as a projection, he shouldn't.
Factual error: Many of the episodes are set in the 1950's or early 1960's, but we always see three-prong electrical outlets in the buildings. Three prong electrical outlets only came into common use in the mid to late 1960's, and were required starting in 1969. While they existed before that, they were only installed in specific use situations.
Chosen answer: Per the Quantum leap page at http://www.scifi.com/quantum/episodes/season5.html. 8 August 1953: An enigmatic leap lands Sam in a Pennsylvania tavern, as his own grown self on the day of his birth. As Al and Gushie work frantically to locate him, Sam befriends a wise bartender (popular character actor McGill, who'd appeared in a different role in the very first "leap") and a group of coal miners. As a host of familiar-looking faces pass through the bar - with different identities than Sam remembers - Sam ponders his life of leaping with Al the bartender, who tells Sam he controls his own destiny. Pressed for more, Al the bartender simply shrugs and says, "Sometimes, 'that's the way it is' is the best explanation." Sam realizes he must right at least one more wrong before he can go home, and leaps back to tell Al Calvavicci's wife Beth (from "M.I.A.") to wait for Al, who will survive Vietnam and come home to her. The closing title cards state that Beth and Al have four daughters and will shortly celebrate their 39th wedding anniversary ... and that Sam Beckett never returned home.
Boobra