Anytime sf::Mouse::isButtonPressed is used, it caused the application to crash. Using GDB to check the error, I get this:
Program received signal EXC_BAD_ACCESS, Could not access memory.
Reason: 13 at address: 0x0000000000000000
0x00007fff969cd310 in IOHIDElementGetDevice ()
(gdb) bt
#0 0x00007fff969cd310 in IOHIDElementGetDevice ()
#1 0x000000010004cbd1 in sf::priv::HIDInputManager::isMouseButtonPressed ()
#2 0x000000010004c7e7 in sf::priv::InputImpl::isMouseButtonPressed ()
#3 0x00000001000486f9 in sf::Mouse::isButtonPressed ()
#4 0x0000000100006285 in main ()
I'm using SFML 2.1 on Mac OS X 10.8.4.