I've got examples of stupid things I've done in practically every human endeavor, but since you asked specifically about ones relating to firearms, here's one of many:
Where I grew up north of Indianapolis it was considered a suburb, but I used to routinely shoot my .22, blackpowder, or a shotgun in my yard and none of the neighbors cared. We had an unfinished dettached garage that I would occasionally test some guns in, shooting into the stud cavity that was filled with insulation. The garage had a brick exterior, I never caught a bounce-back, and it never went through to harm anyone or anything.
Fast forward to my first place after college, one half of a duplex my wife and I rented before purchasing our first home. One night I got the itch to test a .22, so I went out to the garage and fired it into the wall above the garage door. I thought that this should be fine, as the garage was insulted, and even had drywall on the interior. I figured I would patch the holes later.
What I forgot was that the exterior was NOT brick. I fired three shots - all in a nice group - and the next day went outside and saw - to my horror - that they had not only punched right through the exterior wall, but had blasted a rather large chunk of the vinyl siding along with them! Lord knows where they ended up. I was so embarassed and mad at myself I couldn't see straight for a week!
![Embarassed :oops:](./images/smilies/icon_redface.gif)