This was WAY back in my first job as a programmer where I was working on a custom built CMS that we took over from another dev shop. So a standard feature was of course pagination for a section that had well over 400,000 records. The client would always complain about this section always being very slow to load. My boss at that job would tell me to not look at the problem as it wasn't a part of the scope.

But being a young enthusiastic programmer, I decided to delve into the problem anyway. What I came to discover was that the pagination was simply doing a select all 400,000 records, and then looping through the entire dataset until it got to the slice it needed to display.

So I fixed the pagination and page loads went from around 1 min to only a few seconds. I felt pretty proud about that. But I later got told off by my boss as he now can't bill for that fix. Personally I didn't care since I learned a bit about SQL pagination, and just how terrible some developers can be.