If that article interests you, you might find this one even more interesting:

http://zipcpu.com/blog/2018/04/02/formal-cpu-bugs.html

Basically, I was shocked when I applied SymbiYosys to the ZipCPU at how many bugs I found. Bug lists like this are typically company secrets, since no one wants to reveal how many bugs their product has. In my case, part of my success has been my openness.