The audio that plays from the camera when Mr. Butler finds it in Lucas' room does not match the audio of the scene it supposedly recorded.
Lucas was given back his graded essay, where the teacher refused to change his grade left Lucas cold and dry with the given B+. However, once the teacher begins to investigate - he retreats to his home office using the evidence for probable cause against Lucas that which was literally shown given to the student a scene prior without retrieval of it before leaving the classroom: therefore; mistake being the "dismissed" error of providing teacher with the misplaced direction of the essay to "investigate."
While accounting the font to which Lucas used in his essay to that of his framed written Application, the first appearing font of the essay is different and proceeding that is shown the letter font - as it switches back to Luke's essay it is changed to matching the applications font, ignoring the fact the first sight of the essays font from submitting it to receiving it to the reopening of essay at butlers home office had a completely different font.
The teacher gives Lucas a B+ on his paper because he disagrees Lucas's interpretation of the play. This is high school, and Lucas has written a paper well into college level standards. Lucas made an argument and supported it with text from the play. Essentially, the teacher is saying he'll only give an A to views that agree with his own.