In every project there are gotchas, little unforseen issues that must be worked through as you migrate from development to production. In a previous post I brought up transitioning a Model-Glue: Unity application from a Windows development platform to a Linux platform while using two slightly different versions of MySQL. That would be one example.
Another recent project serves as a case in point as well, creating compliant header/nav/footer includes to be used site wide for a small site. Easy work, three or four files, including the stylesheets, with some example usage templates thrown in and a lot of comments for the developers who would use it. Oh yeah, did I mention it had to work on ColdFusion 5?