You might have encountred a full screen application that displays alerts to you, but it goes full screen and another alert goes in the back and you cant click it because the application hides it and dosent allow to alt-tab obviously you now need to either terminate the app or force reboot your computer.
So what to do?
Well you can do a few things:
1. Put comodo defence+ to learning mode and lunch the application this will automaticly allow the alert and allow you to continue.
then try and do stuff that you suspect that can cause an alert in the application, to create more rules to prevent lock-ups in the future.
afterwards close the application and return defence+ to normal (whichever secuirty level you choose).
2. create a rule manually or use a predefined rule that will suit the application needs.