The Sea Devils: Episode Two - S9-E10
Continuity mistake: In episodes 2 and 3, watch the clock on the wall in the Master's quarters. When the Master pulls a gun on the Doctor and the Doctor quickly pulls the door shut, the clock shows ten minutes to three. In the next episode, after the swordfight is over and the Doctor is being led out by the guards, the clock now shows approximately eight minutes after twelve.
The Sea Devils: Episode Three - S9-E11
Continuity mistake: The helicopter sent to rescue the Doctor and Jo changes in mid flight. It begins as a grey Sea King marked SO. In the second shot, it has an orange nose and tail stripe and is numbered 56.
The Curse of Peladon: Episode Two - S9-E6
Continuity mistake: As Jo walks along the castle ledge to escape from The Ice Warriors' room, there is, at one point, a quick shot of a piece of the ledge breaking off under Jo's shoe and falling down the cliff face. However, the shoe isn't Jo's. If you look closely, you'll see that the shoe in that brief shot is more pointed and of a darker colour than the shoes Jo is wearing, and has a strap across the ankle instead of being a slip-on.
The Sea Devils: Episode Six - S9-E14
Continuity mistake: The Doctor and the Master partake in a boat chase. They are both soaking wet when they arrive at land but in the next shot their hair and clothes have magically dried.
The Curse of Peladon: Episode One - S9-E5
Continuity mistake: When the TARDIS falls on the cliff at the beginning, the Doctor helps Jo out of it. In the next shot (model shot) the Doctor and Jo are nowhere to be seen.
Chosen answer: The Master is insane. If he can't control the universe, he'd rather see it destroyed.
Captain Defenestrator