Sometimes the expert has to tell the client that their case probably won't hold up.
As happens so often, the defendant had petitioned for a reexamination of the asserted patent, and I was asked by the plaintiff to help out. The patent-in-suit was a division of an older case, so the specification had a lot of good inventive stuff in it, but the actual claims of this patent concerned ultraviolet-activated fluorescent displays (the black-light ones you see in bars and restaurants that you write on with a fluorescent marker). An interesting idea, for sure, but one that has been in wide use since the 1960s at least. The defendant/petitioner pulled up a lot of prior art, including a Web catalogue from the Wayback Machine (archive.org) that showed the same kind of boards and fluorescent markers being offered for sale a decade before the priority date of the patent. As far as I could see, they met every one of the limitations of each of the asserted claims. As I couldn't find anything wrong with the prior art, I gave the client the bad news, which of course made it a short gig.
After I left the case, they kept on with the review, and the asserted claims were ruled unpatentable by the Patent and Trademark Appeals Board.