Few things are more forgettable than the “anniversary” episodes of TV shows. Sure, they have meaning to the cast and crew, but I can’t imagine that the audience really cares whether it’s the 300th episode of “ER” or the 297th.

These milestone also seems out of sync with the very nature of TV entertainment, a medium that exists in an eternal present. When we watch a really good rerun of “I Love Lucy” or “The Honeymooners,” we’re not concerned with the realities of 1952, but with the timeless quality of the characters and comedy. Some of the more dated reruns of “Seinfeld” make references to mid-1990s phenomena, like the O.J. trial. But the laughs still transcend time.

Reality shows, their very nature, involve constant repetition, padding and narrative spoon-feeding, and they barely hold our attention for a single viewing, never mind sustaining interest for years of repeats. They are very much like sports. Who, besides the most devoted fan, really wants to watch a football game, or “Survivor” or “Idol,” more than once?

So does anybody care about the 100th episode of “Wife Swap” (7 p.m., ABC)? Particularly when this “Swap” is a contrived rematch?

To commemorate the big event, ABC polled fans of the show, now in its fifth year, to pick their favorites out of the 198 families who have endured the experiment in parenting styles and clashing cultural cliches.

Viewers chose the Heenes, the storm-chasing, UFO-obsessed family from Ohio, and the Silvers, a family of performers whose matriarch is convinced she can communicate with the dead. Look for extroverts and fireworks and clashes between two families who have done this all before. True to the reality genre, the 100th “Wife Swap” is thoroughly recycled.

• Scheduled on “20/20” (9 p.m., ABC): Libertarians John Stossel and Drew Carey find fault with the bailout. To steal Stossel’s catchphrase: Give me a break. Before they get to nitpick the government’s reaction to an economic catastrophe, I’d really like to hear these free-market types try to defend the unregulated financial environment on Wall Street and the housing and lending markets that resulted in our current woes. I don’t think that’s going to happen on tonight’s “20/20.”

Why not? Seriously, why should we listen these guys whine about entitlements like universal pre-kindergarten (government: bad) when they can’t explain why a laissez-faire approach to Wall Street regulation allowed tens of trillions of dollars to disappear? Could it be that their cherished notion of unfettered markets resulted in a felonious free-for-all? Or does their idea of “freedom” include our freedom to starve?

• California coeds compete to see who can survive a sleepover in a haunted hospital on “Hot Girls in Scary Places” (8 p.m., E!), a send-up of several reality formats in one.

• Turner Classic Movies presents three classics based on novels by James M. Cain: “Double Indemnity” (7 p.m.); “Mildred Pierce” (9 p.m.) and “The Postman Always Rings Twice” (11 p.m.).

Tonight’s other highlights

• Sam risks all to save Melinda’s mind on “Ghost Whisperer” (7 p.m., CBS).

• Jesse recalls her submarine voyage on “Terminator: The Sarah Connor Chronicles” (7 p.m., Fox).

Cult choice

Tori Spelling stars in the 1996 thriller “Mother, May I Sleep with Danger?” (8 p.m., Lifetime), featuring one of the dumbest titles in the history of TV movies.