My coolest bug fix was fixing XSS and CSRF vulnerabilities. It was the starting of my IT career and when I hear these big names, I used to think that it takes a big brain to fix them. But the solutions were rather simple. My architect told me how to solve them and I made my version of the solution and sent it for his review. He just rejected it and told some enhancements to it. The to and fro of these reviews happened for a week.
At some point I felt, why don't he f*****g do it himself. It would take him about 5 minutes.
Finally my code was approved.
Now when I turn back and think about it, I feel I learned a lot from that exercise.