Continuity mistake: In the first minute of episode three, one of the evil robot mummies attacks Sarah-Jane Smith. In doing so, it lunges forward, misses Sara-Jane, and smashes the "Marconiscope" device, which explodes in flames. However, seconds later, the 'Marconiscope' device is still visible in the next shot, but totally undamaged.
Continuity mistake: In episode 3, when the Doctor leaves the old house to find the dynamite, you can see his hat resting on a chair as he walks out the door. Immediately afterwards you see him walking through the woods with Sarah, and see that the hat has magically relocated itself to being back on his head again. And when he eventually gets back to the cottage, the hat is there, still in the chair as if nothing had happened.