When Mac OS is under extreme load it seems to be fairly easy for applications to miss a message and stop responding. If this happens to finder it can appear as though nothing wants to work anymore. Not even the "Force Quit" option will show up in the system dock. Call me slow but I just found an alternate way to force quit an application that has stopped responding. The Apple three finger salute is "Apple" + Alt + Esc. That brings up a task list and from which you can select the offending process and kill it. Beats using ps and kill!