I have seen it a lot of times nobody is perfect 100% of the time dont care who ya are. And remember he is on a re-do the stress is worse the second time around makes missing some little thing easier. Every time you take something apart you have the opportunity to stir it up. :thumbsup:
for instance I see mass air flow sensors get dirt and fuzz in them quite often from someone simply changing filters and letting a chunk of dirt or debris loose from around the filters seal and that turns the check engine lite on - po171 its common.
I personally let a metal mass air flow meter mounting bushing spacer loose into a 280z intake one time when I was replacing the vane type meter. And it went clear into the cylinder, I had to drill the plug hole and fish the chunk out and install a helicoil plug repair insert to fix the car **** happens to all of us.