Common Faults: Bar Finishes Out Front in Jerk
We have all seen it. The bar flies overhead appears to be locked out but falls in front of the lifter. If you have not read Common Faults: Incorrect Jerk Dip or Common Faults: Balance on the Feet I suggest starting there as these are usually the issue of a misplaced barbell. The dip and drive is what gives the bar it's direction.
Assuming the dip and drive are correct other errors could be:
The bar slides off the front rack
Lifter pushes the bar forward after the drive - usually to move the bar around the face instead of the face around the bar. (When over body weight the lifter will usually travel back)
Not reaching the front foot
Fear of the bar being overhead