Lorna Mitchell has a new post to her blog today talking about static methods and how to use them correctly in your code (as discovered accidentally in her own code).
I was confused recently to realise that I had accidentally called a static method in PHP dynamically from another part of my code; I expected PHP to output warnings when this is done. On closer inspection I discovered that: static functions can be called dynamically and dynamic functions generate an E_STRICT error if called statically.
She illustrates with some sample code that, when run with E_ALL and E_STRICT throws a warning from the strict side about calling a non-static method statically. She also talks about why it throws this warning for the non-static call on a static method. She also explains why, when a static method is called dynamically, no warning is thrown.