I just saw the episode "Like father, like son" on DejaView (season 8, episode 12) and noticed something funny.

While Jack is with Angelino in the kitchen of Jack's Bistro, his father steps in and says "I'm just here to get a glass of water". Of course, being in a studio, the pipes are fake. The solution: You see Jack's father simply picking up glass already full of water at the bottom of the sink :lol:

There must be tons of these hard-to-notice "bloopers" like that.

Yeah I've seen many of mistakes such as studio stage lights visible in some parts.

The main door being closed then open in some shots.

A good one is on season 5 "Dying To Meet You" as Terry Kiser is pushing Janet out of the kitchen there is nothing but a huge wall behind them.

I think the biggest one is when Terry drops a bag of groceries in the fake kitchen and, while she steps out, you see a hang picking up the bag. Like if they couldn't leave the bag there for a few more seconds!

A couple of instances where the front door is both open and closed

"It's Only Money" - when Jack is describing to the police officer items that may have been stolen.

"Jack's 10" - when Jack and Denise are breaking up.