Recently we tried the "What is one thing you learned from a project that went wrong?" and what to say... it went really well.
At the beginning all were still quite hesitating, but after the first two "What went really bad for me in a project / job and that's the thing I learned" speeches, a lot of input came in.
There was a lively exchange of experiences and it was noticeable that the atmosphere was very open, supporting and relaxed. A kind of community feeling arose in the sense of "oh, he / she had problems in a project too and that is a great tip" we are all in the same boat and nobody is perfect.
It works in a group of newly assembled project teams but also with longterm colleagues - everyone has failed in a project before, learned a lot, brings a bunch of experience with him / her and everyone is happy to take away lessons learned.