I had a nice rain jacket that I put in my ortlieb saddle bag. If you don't close that right, it just opens right back up. Silly fastening system. Unfortunately, someone in the crowd lining the street thought they needed my jacket more than me. Fortunately, the weather was fine for the rest of the day and a half of the ride, so it wasn't an immediate problem. I only buy cheap rain jackets nowadays
I also lost a 10mm wrench because of that bag. Of course, 10mm wrenches are all temporary, it's the wrench that is the most difficult to find, so they must walk off on their own