Probably the biggest "nightmare" we had was a local publisher, and this was back in around 2000, who wanted to put their magazines on the web. They were highly targeted "yellow pages" for tourist towns, so they really wanted to take their existing PDF artifacts that they send to the printer and make them available on the web as clickable hyperlinked things inside a web based viewer.
So we proposed making a proof of concept, we spent a few weeks working on splitting the multi-page PDF, making a viewer that would pre-cache pages, allow you to zoom and scroll, and defining the bounding boxes for some pages so that you could click and go to the homepage of the advert you were clicking on. We demoed it for them and they liked it, but they said they had decided to put that project on hold. I explicitly asked them if they were happy with what we had built and they said yes.
Around a year later I get a series of calls from them and from a new web development shop they have hired, saying that they wanted a refund because the work we did "our new consultant says they could have done in 10 minutes". My feeling from the conversations I had with them at this point was that the new developer was going to get paid out of the money they got back from us.
I don't remember what our final resolution was, we always worked very hard to make our clients happy. We really didn't like that they had said they were happy with what we had done, then a year later some third party called us telling us our work was crap and we should give a refund. And the whole "We could have done it in 10 minutes" was not a very good tactic, it just pissed us off.