Super Plot hole: A time machine was introduced in the series. The introduction of such a device completely destroys the logical thinking of all the characters in the Harry Potter series.
Time travel means that good guys and bad guys could use time travel to solve their problems. Voldemort could come back in time and not kill Harry Potter and control humans to throw grenades in his house. Harry could come back in time to save his parents by telling them when Voldemort is going to kill them. Dumbledore could come back in time and accio a big wood furniture into little Tom Riddle's face. The possibilities are endless.
Instead, Hermione and Harry use the time machine to go back 3-4 hours in the past. And they are acting hastily even though they have all the time they need with a time machine. The simple fact that Hermione can get a time machine in order to have more classes show how easy it is to get a time machine. This is completely retarded.