When it shows Sam walking into Hilla's room the reflection in the mirror shows the door jam is on the left and handle is on the right. When the camera pans around to reveal it. It has the door jam on the left and handle on the right. Which means it wasn't a proper reflection in the mirror.
When Sam Beckett reads Hilly's diary, the voiceover, apparently her diary entry, talks about July 4th and how she had a fight with someone. However, the German text in the diary says nothing of the sort - instead, one entry talks about a trip to the zoo with someone named Anton and the other actually places her at the Chiemsee on July 2nd.
When Lyle confronts Melvin in front of the Mirror, you can tell by the angles that the mirror is tilted. You can also see that the actor, Marvyn Byrkett, is rushing to "hit his mark". In addition, you can also see Scott Bakula's shoulder for an instant.