Noticeable errors, mistakes or goofs

I don't know if this is a mistake or goof but Larry once had a girl at his place named Diane whose name he kept forgetting and a few seasons later had a young sister on named Diane.

In the episode were Jack's dad is on, we learn that his name is Jack Tripper Sr. but wouldn't his name simply be John Tripper to tell the two apart?

In season 8 episode 3, Jack hides money from the ATM in the couch cushion. The kids get a new couch, courtesy of Mr. Ferley. In season 8 episode 4, the old couch is back. What happened to the new couch?

A couple of minor ones regarding Janet.

In "The Loan Shark" (Season 4), it's said she can speak moderate Italian. In "The Odd Couples" (season 8), she's flustered about playing a foreigner, and doesn't know what country her "character" should be from. Wouldn't Italy be an easy choice?

In "Teacher's Pet" (season 5), Janet learns to cook, and the tag involves her making a decent meal that makes Jack quietly jealous. In "Baby, It's Cold Inside" (season 8), Janet says "I've never cooked chicken before!" Terri responds, "You've never cooked anything before!" Janet nods and agrees.

In the "Charming Stranger" epsisode, Jack's testicle makes an appearance. It wasnt seen before of after that. It's similiar to the disappearance of the Cunninghams' oldest son Chuck from Happy Days.

Since I've always loved languages, I've noticed THIS:

Early on it was established that Janet spoke SOME Italian when Jack was teaching the Italian woman how to cook, BUT in a later episode when Terri talks Larry and Janet into being foreign house guests they were they were trying to decide on what country to be from... It seems Janet would have said something like, "Well I already speak SOME Italian, why don't we be from Italy?"

Well they finally agreed on France... And Larry says, "But I don't speak French!" And Janet replies, "Neither do I, but we'll fake it and they'll never know!" When I'm ALMOST sure it was established that Janet spoke SOME French in an earlier episode as well, I THOUGHT it was the episode where Janet and Mr Furley go to a French restaurant, and Janet looks at the menu and says something like, "I did take French but it's a little rusty!"