Audio problem: When Greg and Peter were in the family room helping Marcia write a letter to Davy Jones, Peter suggests telling him that he won a $10 million sweepstake. Then as they walk into the kitchen, you hear Greg saying "A $10 million sweepstake" and Peter says "$20 million?" It is clear that he wasn't really talking, because the sound of his voice is very distant, although they were only in the next room.
The Hustler - S5-E21
Audio problem: There's an ominous mic echo when Bobby is talking with Mike's boss at the pool table, indicating this was filmed on a sound stage.
Audio problem: There's a very echoey essence all throughout the episode like they're shooting in a cave.
Continuity mistake: Mike has just pulled into the driveway and honked, as the family waits excitedly for him to come in and share his great news. Then, a second later, he's coming around the counter to greet everyone, instead of a car door or patio shutting.
How to Succeed in Business? - S4-E19
Plot hole: At one point Peter admits to Greg he was fired. In a later scene, Peter tries to talk the parents out of going to the bike shop (where they think Peter still works), and soon after the parents ride new bikes and see Peter in the park and admit to him that his boss told them what happened. Carol said that Greg filled them in as well. This is incorrect because before leaving the house, they didn't know of the firing, and went straight from the bike shop to the park. Greg never told them.
Suggested correction: Just because we didn't see it on camera doesn't mean it didn't happen, it's known as "implied action."
That's not really the case here... the sequence of events explain it all. It went from going to the bike shop to find out what happened to the park. Even if they went home in-between, there would still not be a reason to have to question Greg, since they know what has happened at this point.
To build on the implied action point that the user stated, we do not know what happened off-screen so it's not impossible to believe that Mike and Carol went home after learning about what happened from the boss to talk to Peter and then have Greg explain where he may be and why. Peter did tell Greg about feeding the pigeons in the park so it tracks.
Other mistake: When Mike is talking to Mr. Crawford and the operator asks him to deposit another 10 cents, Mr. Crawford said, 'Did I hear an operator ask for 10 cents?' Only Mike would have heard that recording, not Mr. Crawford (he didn't have the pay phone).
Revealing mistake: When Carol goes to the dress rehearsal, she is wearing one outfit when she is talking to Mrs. Goodwyn, but then when she goes upstairs to the girl's bedroom and talks to Marcia, she is wearing a totally different outfit.
Suggested correction: Carol was talking with Ms. Goodwin during school hours, and when talking with Marcia about it in the bedroom, it was presumably evening. It's possible Carol may have taken a shower in between that time frame offscreen, thus changing her clothes.
Other mistake: Near the end, when the police chief "sees" the fake UFO, he picks up the phone and dials 3 numbers and immediately speaks to said caller instead of the operator or asking for an extension.
Suggested correction: Maybe he dialled 911 as an emergency. He even told the person on the phone to "alert the defence command immediately" (or something like that).