Why we inspect telescopes: secondary mirror glued on incorrectly and a bent screw.

While attempting to collimate a Newtonian telescope (Explorer 200 P), we discovered—after unsuccessful adjustment efforts—that the mirror had been glued on crookedly and the retaining screw was bent. Naturally, one could spend ages trying to collimate it without achieving a satisfactory result.
We re-bonded the secondary mirror and replaced the screw. After readjusting everything, the result was perfect: the telescope collimates flawlessly and is practically on its way to the customer.