A borescope plug being left out!....but those items are on the independent inspectors list of things to sign off....how did that happen?
Still, it never ceases to amaze me how it can happen...
In much the same way as pilots can land with gear up, or attempt to take off with incorrect flap settings etc. Procedures and checks not correctly followed, or the procedures themselves are flawed.
Unfortunately we humans are fallible, whether we are 'on the tools' or 'driving' the aeroplane.