Most of the systems I worked on, were legacy, badly-engineered systems. I worked for five years "maintaining" (or trying to maintain) a commerce platform made with pure PHP, an old version of PHP that couldn't be really updated. The platform depended on an external API that's constantly changing (as we speak): changes that couldn't be reflected on this platform as it'd imply a complete rewrite of such system. No documentation, no worry about good practices, "just keep it" (while dealing with angry customers, as I was also responsible for internal support intricacies). Good thing I left, although I miss that money for... I dunno... surviving, as I have no prospect of being hired any time soon. I worked for 10+ years (cumulative experience) as an IT professional, but I'm sincerely thinking of abandoning my "career" for a simpler job. I love computers and I love math, but I hate being a cog inside a broken machine.