off the top of my head:
- header flanges dont line up with the head
- header flanges too thick for o.e. exhaust studs
- header leaks at flanges
- 02 sensors placed in wrong spot/angle
- header does not clear front control arm bushing
- header does not line up with o.e. section 2
but if you can deal with these, they are cheap enough to make work-as long as the welds dont crack.