Timing cover pitting.

I normally not only clean the area I am going to disassemble as good as possible to avoid stuff falling in,
If the work involves the distributor/Harmonic Balancer I also set the balancer to TDC on the compression stroke, verified with the ignition rotor and mark the location of the distributor housing before taking anything apart.
No wondering where anything was.
And if it is in the car and has a battery, disconnect both terminals, starting with the ground wire.