Theory:Doctor Who television discontinuity and plot holes/Arachnids in the UK


 * At the end of the episode, music is used to lure the spiders in the hotel into the safe room. However, early in the episode it's established that spiders are all over the city, so there must still be giant spiders around Sheffield.
 * Spiders of that size are physically impossible, even the smaller mutants would be too heavy to support themselves much less crawl around on the ceiling, and they would suffocate because of their respiratory system. The atmosphere isn't oxygen rich enough for insects that large anymore.
 * When the Doctor traps a spider with a line of salt in the flat, the spider is easily large enough to step over it, or it could just crawl on the walls to reach the Doctor.