Once in my apartment when I was 21. Two guys broke in to try to attack my roommate while he slept. He was a big guy and liked to run the bars. These two waited till he went to bed because they thought they had a chance against him then. Got out my Mossberg and "diffused the situation" They left and we called the cops. Small town, we knew them by name and so did the cops...
The other time was a stray dog. Nastiest animal I have ever seen. It was between me and my car in my driveway one day. It was sick, malnourished, missing hair. It saw me and barred its teeth and began to growl. I showed it my GP100 and it ran. The dog seemed to know what a pistol was when it saw it. The dog came back later, chased my girlfriend from her car into the house. I shot it twice in the head with my AR 15.
Stopping power: The power some guns have to make you stop what you are doing and get your wallet out.