Deliberate mistake: Tim babysits his 4-year-old niece in the season 7 episode Say Goodnight, Gracie, but in season 8, which takes place the following year, Gracie and her twin sister Claire have been aged up to around 8 years old.
Revealing mistake: When Tim gives Jill the framed photo of the two of them on their way to the Doobie Brothers concert, you can tell Tim has just been photoshopped into an existing picture of Patricia Richardson, as he is slightly out of focus despite the fact that they are standing next to each other. (00:13:24)
Revealing mistake: At the end of the episode before Tim opens the front door for Julie Zwicky, you can see the actress' silhouette in the window pane for several moments before she rings the doorbell, showing her to be waiting for her cue.
Tanks For The Memories - S5-E15
Continuity mistake: The boys had different haircuts in the previous episode. Randy had just gotten a haircut, and in this episode, the kids are younger and have their longer hair right back. (00:05:50)
Continuity mistake: When Randy takes the phone from Mark, as he starts to turn away he lifts the phone to his ear. The angle changes and he is now holding the phone in front of him as he walks over to the magician's box and jumps up on it.
Continuity mistake: In one of the episodes, Tim goes out to talk to Wilson. When he gets to the fence he picks up a football. When the camera cuts to Wilson, then back to Tim, the football is gone. Then the camera goes to Wilson and back to Tim. He has the ball again.
Revealing mistake: Anytime the kids are shown running through the backyard, it sounds very obvious they are running on a wooden set floor instead of real grass-covered ground.
Baby, It's Cold Outside - S1-E18
Revealing mistake: When Wilson and Tim start to talk they sit down on some rocks. If you look at Wilson's foot you can see the sheet of fake snow caught under his foot.
Chicago Hope - S5-E9
Visible crew/equipment: At the beginning episode 5.9, "Chicago Hope", the camera and several studio lights reflect on the glass door behind Tim as he sits at the dining room table.
Let's Go To The Videotape - S4-E7
Continuity mistake: The conversation Tim and the others have at the hardware store becomes a lot different when Jill sees the (supposedly) same conversation on videotape. First of all, Tim's remark the "Jill's speech was so boring" has become "Jill's speech was so DULL". Secondly, Eddie's entire tale on how his wife wanted to change the fabric in the sun room is missing from the tape. And third, when Tim is talking about Jill's psychology studies, the expression "these terms" has become "these USELESS terms".
Continuity mistake: When Al and Tim are in the bar, watch Tim's and Al's glasses topped with beer foam. In the first shot there is a lot of foam, yet in the very next shot there is no foam.
Revealing mistake: Towards the end of episode 2.18, "Bye, Bye Birdie", Tim drops to the ground in the back yard while trying to avoid the helicopter. When Tim gets up, you can see the grass (carpet) move and wrinkle up.
The Karate Kid Returns - S6-E14
Other mistake: When Tim and Wilson are talking about The Beach Boys song lyrics, Wilson says he wrote a book called 'The Psycho-Physiological Indices of Amorous Connections Among Termites of the Southwest' that sold four copies. In an earlier episode called Wilson's Girlfriend, the woman Wilson dates also says she wrote the same book with the same number of copies sold.

Visible crew/equipment: At the very end of the episode, the top of the set and crew lights are visible on the top on the screen.
Ye Olde Shoppe Teacher - S4-E10
Revealing mistake: In this episode, every time Wilson places a log on the block before splitting it, you can see a dark line on top of the log, showing where they were pre-cut.
Revealing mistake: When Tim is falling, crawling and rolling on the snow-covered ground in the final scene, none of the snow clings to him or his clothing. He does not even get damp, even when he is lying on his back in the same spot for several seconds.
Visible crew/equipment: As Jill is describing how, when she stops at an intersection, the "car guys" revv their engines, a crew member can be seen reflected in the window behind her, moving across the stage from left to right.
Karate Or Not, Here I Come - S2-E19
Continuity mistake: When Jill is standing up to the other woman at Mark's karate class, she asks her "Will you stop calling my son 'Stick Boy'?" As the other woman replies "Then feed him something", her hands are on her hips, but when the angle changes, she has her right hand on her chest, with no time to move her hands between shots.
Continuity mistake: After Tim smashes Al's display cabinet, it cuts to Tim, then back to the workbench and the wrecked cabinet. The pieces of china and the pieces of wood on the bench differ in the two shots. In the first shot, there are two whole teacups, one whole plate and a bit of a shelf on the bench (the shelf is on the right side of the screen), in the second shot we see a broken tea cup, one broken and one whole plate, and the piece of shelf has disappeared.
Ye Olde Shoppe Teacher - S4-E10
Continuity mistake: At the beginning of episode 4.10, "Ye Olde Shoppe Teacher", Tim's lopsided toolbox that he made turns 180 degrees as Jill pushes it away. As Jill and Tim are talking, it turns back. Then it turns 180 degrees again just before Tim briefly leaves the room.
Answer: No the guy sent in a picture of himself because he thought he looked like Al. So they brought him on the show as his brother.
Bowling255