I can't count how many times I've come across even the smallest issue causing massive problems in different browsers. Sometimes, it's just a simple fix requiring something like quick polyfill, but most of the times it ends up being an endless rabbit hole of botching, fixing and testing. There have actually been times where I've fixed something in such intricate ways that it made me question reality.
And don't get me started on maintaining old codebases. It seems like code just rots if you leave it for a few years.