It is so natural these days to use the following Visualforce code to control the displaying of something wrapped in a pageBlockSectionItem.
<apex:pageBlockSectionItem rendered="{! clazz != null}">
<apex:outputLabel value="Hellow World!">
</apex:pageBlockSectionItem>
Even though the syntax {! clazz != null} looks strange, it is easy to read once getting used to the convention that first exclamation point in the curly brackets means the expression starts from there. Clearly, what the above expression tries to do is to only render the “Hello World!” text when “clazz” property in the related controller is not null. However, this does not work correctly if the Visualforce page is at version earlier than API 26 (Winter ’13 Release). It seems to always return true (at least it always renders the text) in a page with API version earlier than 26. See Winter ’13 release notes Page 202 for more details. I recently spent 2 hours or so debugging a defect that used such expression and only got to resolve it by changing the version number of the Visualforce page. This is the type of defect that cannot be covered by unit testing on controllers only. Manual testing or automated testing like Selenium is necessary.
By the way, the following functions used in Visualforce page to evaluate null property works correctly all the time:
<apex:pageBlockSectionItem rendered="{! NOT(ISNULL(clazz))}">
<apex:outputLabel value="Hellow World!">
</apex:pageBlockSectionItem>
Force 746 