Great idea. I always learn so much by my own and other people's mistakes.
1. I used to girth hitch the little cord of my point and shoot camera into a sling so as to prevent from accidentally dropping it. Then one day about 5 pitches up on SEWS I looked down to see that I had clipped into that little camera cord as my tether at the anchor. Someone I grabbed the main sling but instead of clipping into the end of it I clipped into the carmera cord at the end. Needless to say I don't ever tether my camera by a sling anymore.....
2. I had a friend fall and die while climbing with a new partner on a long multipitch route. They were climbing above their ability but their new partner didn't realize it. At the time, I was climbing with a lot of new people on bigger routes as well. I realized that I could have easily been my friend's "new climbing partner" in that scenario. It is so hard to tell someone's actual level of experience from a hike up to a cliff/mountain. I had previously thought that as long as I was solid at whatever grade we were doing it didn't matter. But after a lot of reflection I decided that wasn't true. If I was climbing with someone of less ability they could want to "hold up their end of the partnership" by leading, routefinding etc. But what if they are way in over their heads, and I don't know it because I don't know them as a person or their past experience. Now days I want to spend a day cragging or just get to know someone as a friend before getting into a big route together.