Impossible question
You often don't know that your code is buggy until it's exploited and while static code analysis may pick up some obvious flaws, most bugs are waiting to be discovered as we've seen over the last few years.
More important, therefore, is to list common gotchas: memory management, handling of untrusted data, equivalence tests, etc, and the strategies used to deal with them.
Also, things don't stand still. It's no accident that PHP has so many CVEs filed against it: up until a few years ago it was the goto language for web development and unattracted a lot of untrained people as a result. Many of them have since moved onto other languages (JS for web and app development, Python or R for data analysis). But threats have also changed: SQL injection is perennial but, hopefully, less of an issue today, but now we have more attacks on transport and handling.
Biting the hand that feeds IT
