The cause of the worst, most frequent, and recurring bugs is from having a poorly conceptualized or structured implementation. After a product has been built th

The cause of the worst, most frequent, and recurring bugs is from having a poorly conceptualized or structured implementation. After a product has been built this way it is very difficult and error-prone to change it into something more suitable. Now if you imagine a waterfall project that delivers the entire feature-complete v1.0 product, I can imagine that this is much more likely given that real-world volume testing happens after shipping/launching it. With a product that evolves with its user base, acquiring features as users gain experience to demand it, all the while being used and exercised by the user base, it will also have bugs along the way, but there may be many more opportunities for course corrections that are achievable without a full rewrite. There's is also every combination in-between and what makes sense depends on the specific subject area and users. Regardless of which approach one takes, quality/lack-of-bugs depends on making it a priority and sufficient investment. i.e. it is not only/predominantly based on process, it's in the details.