The NIH-developer. I once inherited a project where the lead developer hated third party dependencies, so he rewrote large parts of the libraries that we used, only to get it a couple of milliseconds faster (for very particular use cases only). He creates something like anonymous functions before PHP 5.3 by using variable variables (!).
It took months to understand all of his dirty quirks and all hacks made it almost impossible to upgrade to PHP 5.5, while PHP 7 was already available. Imagine what a performance gain that would have had.