I just did a really dumb one.
Timing chains on an '01 4.6 F150
already had the front cover bolted up, p/s pump back on and started installing the balancer, when lo and behold I look up a the battery and what do I see?
The friggen reluctor wheel for the crank sensor. Arrrgghh!!!
I then remember picking it up off the bench, cleaning it, and placing it on the battery. I was going to put in on the nose of the crank when I returned from lunch.
DUH!!!