Other mistake: The clocks as we learn later in the film stop at 03:07 due to that being the time the witch died. However, the grandfather clock we see for the first time has stopped at 03:07. We then see it stop at 03:07 the next night too. However the minute hand is in a different spot to when it first stopped - it should have stopped at precisely the same point. (00:13:05 - 00:18:50)
Other mistake: When we first see the Warrens' home, the title card spells it Warren's. As it's the home of more than one Warren, the apostrophe should be at the end. (00:15:30)
Answer: It is explained in The Conjuring 2 that Lorraine has a premonition of Ed being impaled.