Deliberate mistake: In the very first episode, the kids are fighting and arguing all throughout the house. Claire then asks Cliff, "Why did we have 4 kids?", referring to Denise, Theo, Rudy, and Vanessa. In all of the following episodes, they actually have 5 kids. I guess they decided to add Sandra (the oldest Huxtable) later down the road.
Deliberate mistake: Several shots have been taken from Series 1, including the group of guards firing in the hangar bay, and various shots of the shuttles during the chase.
Thomas and the Sounds of Sodor - S16-E11
Deliberate mistake: When the engines are performing "The Sounds of Sodor", Emily, Thomas, Percy and Gordon have the correct whistle sounds but at a lower pitch. At one point, James has Emily/Henry's whistle sound, Mavis has her old "New Series" horn, and Thomas has James' whistle sound.
Plane Death - S1-E14
Deliberate mistake: When the bad guy goes to smash a table onto Michael Landon notice the plate of glass in front of him, visible. The glass is there so Landon doesn't get hurt and to make the table break into pieces in front of him.