Writing the HTML and CSS code for an email signature using some existing examples. Not only were those examples using XHTML4 (or older, with inline CSS) but the code smell was really strong in it. After refactoring/cleaning the whole code, I had to make sure it worked on Outlook 2011, guess what... It uses an old Trident renderer so most of the CSS3/HTML5 things I wanted to use couldn't work and the rendering wasn't the same as on IE9.