I don't know anything about Iphones, but on my landline I have the choice of switching my blocking system on or off. If switched off all calls will come through, including blocked numbers.
Do you have anything like O2's TU-GO installed which allows you to use your home wifi to make and receive calls if time mobile signal is weak? I have found that this will still allow blocked numbers to call as it is essentially bypassing the 'phone' element of the iPhone. Otherwise, not the foggiest!
No limit to blocked numbers on an iPhone, you haven't by any chance, managed to flag the number as a favourite as well have you, that might let the calls through. I'd suggest unblocking the number and then re-blocking it the next time it calls just in case something odd happened when you blocked it. As I said the only thing I can think of is some kind of calling via WiFi app which is allowing the calls through.