Continuity mistake: When Dylan leaves the Angels, she returns the stuff that she has borrowed throughout the years. This includes a letter. Look carefully at the position of the writing on the letter. When Natalie and Alex find the box just after, the writing is upside down.
Continuity mistake: When the bombs go off on top of the rooftop, Madison's telescope has disappeared.
Answer: It is making fun of the show CSI. Even still, there are ways of telling if someone had a knee problem by the way they walk and put pressure on the foot. Being able to tell if it was a torn tendon, I don't think so. Check out the way people run and you can tell if they have hurt something in the past because of which leg they favor.