Plot hole: How did UNIT soldiers manage to get the TARDIS through an ordinary door into the Doctor's dorm in the research centre? The Doctor's labs in the various UNIT HQs had either double doors or a removable partition wall. The research centre is supposed to be an ordinary country house with single doors and solid walls. The TARDIS is larger than the average wardrobe and, unlike most modern wardrobes, doesn't come apart so it can be got through doorways. The TARDIS is known to be a "Type 40 capsule"...but there's never been any mention of the TARDIS being an IKEA-style "flat pack self-assembly" time machine...
Audio problem: When Irongron fires his first gun, the sound is heard before the gun actually fires.
Other mistake: After Linx plants his flag in the ground claiming Earth, Irongron and Bloodaxe look offscreen for no reason, as though the shot is finished.
Factual error: How did potatoes come to appear in a 12th century kitchen? The potato was unknown in England until Sir Walter Raleigh brought them from back from the Americas on one of his voyages in the late 16th century, at least 400 years after the period in which this story is set.
Revealing mistake: The explosion of Irongron's castle is obviously stock footage of a quarry explosion.
Revealing mistake: How did Rubeish's glasses manage to improve his chronic shortsightedness when they are obviously plain lenses?
Chosen answer: The Master is insane. If he can't control the universe, he'd rather see it destroyed.
Captain Defenestrator