Yes, this is a sad truth. ESPECIALLY if it's an installation with which you've had little contact.
This is an excellent sort of problem to use the diagnose-then-options model. Small fee (say, $75, or free as part of annual KAA), and then offer 3 options to handle. ONE of the options is for the customer to do it himself (point to the Help). The next two option deal with you doing this, but the important part of them is stating what problems/support you will handle as part of the option. (There is NO support with the DIY option.).
One's pricing on the other 2 options is both value-based and insurance-costed. That is, you know once in a while you will get tangled, but most of the time you breeze through, spending far less time than the price would imply. On an actuarial basis, you still make good money despite the occasional ""loss.