I've had this happen a few times now. I add a class to an element which already has a class ie: elementA =class 1 + class 2. I then style class 2 and it all appears to be ok until I go to a different page where class 1 is present and what was supposed to be the styling of class 2 has been added to class 1 completely messing up my design. If I select elementA with class1+class2, class 2 appears in the classes list but not in the selector.
The worst thing is that in a few occasions I've completely destroyed work that was finished without even knowing it until a long time later which makes it extremely hard to troubleshoot.