Beware of styling changes after SP1

Recently SharePoint 2013 Service Pack 1 was (re)released. Most companies are planning to installed this update, but before it reaches the Production farm lots of testing is done to make sure everything still works as expected. In my current project the customer has installed SP1 in their test environment and we have encountered several changes we needed to deal with.

Different Styles

Our intranet project has a custom masterpage and stylesheets. The header of this intranet project looks like this:


It has a black header with the logged in user all in white. After installing SP1 we noticed that some pages changed to this:


After some digging into what gets loaded, it seemed that after our custom CSS the CoreV15 CSS got loaded as well now. What component caused this, was our next question? It was the NoteBoard webpart.