Theory:SJA television discontinuity and plot holes/Mona Lisa's Revenge


 * The International Gallery is located in London, but outside you can see the bay mark for Buses is in Welsh "Bysiau" on the road.
 * Sarah-Jane is surprised when she hears about the trip to the gallery on the news, but unless there's forgery or law breaking going on this doesn't make sense. Since Luke and the others are minors, the school would have had to send out permission slips to go on a trip, (even if it is in the same town, because they're being transported by another adult) so unless Luke faked Sarah-Jane's signature, or the school is breaking the law, she would have known about the trip or Luke couldn't have gone without permission from his parent/guardian.
 * Given Luke's temporary irritation with Sarah Jane during this story, it's not unreasonable that he did forge her permission. Some schools also operate a "catch all" consent system for educational trips that take place within a specified distance to the school - usually within the boundaries of a town/city or district. If Sarah Jane completed one of these forms at the start of the school year then they would not have needed an additional consent form for the gallery trip. The school would have sent a letter home to Sarah Jane to inform her, but Luke could have easily hid this.
 * Note also that at no point is Sarah Jane upset about issues such as forged permission slips, only that she wasn't told that Clyde had won the contest. Given her upset over a mundane issue such as Luke cleaning his room, if permission slips had been a problem, she would have stated it. Also, Mr. Chandra, being close friends with Luke, not to mention a neighbour, could have waived the form or allowed Luke a "I'll get it signed later" excuse.


 * If the highway man can not speak because he was not painted with a mouth, then how can the Mona Lisa walk, when she wasn't painted with legs? Also, how can the highway man fire bullets, when they were not painted?
 * That was Clyde's assumption. It was never shown or proven that the highway man had no mouth. He simply didn't confirm Clyde's assumption. He may have just not wanted to talk to him.


 * Why does the Mona Lisa speak with a northern accent, when she was painted in Italy and has been displayed in France for centuries? And why does she use phrases like 'kiddie winkies' and 'a bit flash'?
 * It's possible that she heard British tourists in the Louvre with the accent, and decided she liked it better than the one she had been using.
 * To expand on this, the episode establishes that Mona Lisa had been conscious for 500 years, so presumably she picked up accents, modern slang, etc. from visitors, security guards, etc. That said, considering her most recent home had been in France, you'd have expected her to speak with a French accent, and perhaps not in English at all. But perhaps she was a quick study.


 * The events of this story contradict TV: City of Death in which it's said at least seven copies of the Mona Lisa were painted by Da Vinci (six of which bore the Fourth Doctor's message "this is a fake" on the back).
 * There's nothing to suggest that, as Mona Lisa's sentience did not manifest itself for five centuries. The episode does however raise the question as to whether the six forgeries (all painted by Da Vinci) were also sentient.


 * The "flesh and blood" Mona Lisa looks quite different from the painting.
 * Paintings are never 100% representations of the original subject. Perhaps the sentient paint took the form of what the original model actually looked like.