Most of us have done this: Either your partner bails and we go to the crag anyway, knowing that SOMEONE will find pity on you and let you climb with them. Or you get to the crag with your buddy and see some sad, forlorn looking solo in the parking lot with that "my partner bailed on me" look and you take them in.
I have met some really cool people and had nothing but good times doing this. I once climbed 5.9s with a very patient 5.13 climber who taught me a lot about moving and resting on the rock. Of course a stranger NEVER belays my kids. Having spent 5-ish years in law enforcement I have a decent sense for trouble (wife calls it being overly judgemental) and I can usually decide in a few minutes if someone knows what they are doing. Most of the time the folks I climb with are way better than me so its not an issue. BUT, I could imagine some scary movies starting that way too.
What is the best/worst experience you have had "going potluck" at the crag?