I used to work at a small company that made a SaaS product for a niche HR function. One of our largest customers, a bank whose ATMs you likely see every day if you're in the US, decided to drop our service in favor of a solution built in-house. We were disappointed, but had to let them go.

Then, about 2 years after they'd canceled, our support department got a strange email from a user that was having problems. At first, the support rep didn't understand the issue and asked the user for screenshots of the problem. Sure enough, the screen shots looked exactly like our old product, which had been decommissioned over a year prior. It even had our support email address and phone number on it. It turns out the version of the software they developed in-house started by scraping our site, renaming all the .html files to .aspx and then making the dynamic parts be data driven.

When we called them on it, they basically threatened to use lawyers to put us out of business if we pursued the issue, so I guess our management decided to drop it. We did make them change the support email and phone number.