Plot hole: When Malfoy gets into trouble while trying to catch Harry with Norbert, Professor McGonagall dismisses his claims as "utter rubbish", and a "cock and bull story." To prove his story was true, Malfoy could have shown her the letter Ron received from his brother Charlie (which Malfoy accidentally received when he borrowed a book from Ron), which detailed their plan to deliver Norbert to Romania.
Plot hole: On page 56 of the original UK edition, Harry overhears a witch complaining about the price of something as being "seventeen Sickles." There are seventeen Sickles to a Galleon, so a witch who is familiar with the Wizarding world (as she obviously is, given that she knows that price to be unusually expensive) would say "one Galleon" instead. This was changed to "sixteen Sickles" in later editions for this reason. Page 72 in the American version.