I posted on another thread my experience the other day where a D head ****ed off my local MOT tester who is a really decent bloke and would never be unfair. You could get yourself into a situation where you end up going to a testing station where there are a bunch of jobsworths working. In this case the bloke had come back for retest and a service, he had bought some ball joint boots and got annoyed cos they wouldn't fit them as they had had too many experiences where peeps had bought sh!t ones from China for them to fit that didn't do the job and then came back and moaned at them. So, yep, he ffed off to another testing station. We all laughed as we knew he wouldn't get better service anywhere else and any work he paid for would be more expensive.
Not saying you would deliberately p"ss any one off, but it could happen.