Few people know that we tried to start something pretty similar to Rethink Priorities in 2016 (our actual founding was in 2018). We (Marcus and me, the RP co-founders, plus some others) did some initial work but failed to get sustained funding and traction so we gave up for >1 year before trying again. Given that RP -2018 seems to have turned out to be quite successful, I think RP-2016 could be an example of a failed project?

Carrick Flynn's congressional campaign just failed.

https://forum.effectivealtruism.org/posts/Qi9nnrmjwNbBqWbNT/the-best-usd5-800-i-ve-ever-donated-to-pandemic-prevention

the longtermist entrepreneurship incubator still seems like a promising project to me, though difficult to execute.

I will give an example of one of my own failed projects: I spent a couple months writing Should Global Poverty Donors Give Now or Later? It's an important question, and my approach was at least sort of correct, but it had some flaws that made my approach pretty much useless.

See: How many EAs failed in high risk, high reward projects? https://forum.effectivealtruism.org/posts/JtE2srazz4Yu6NcuQ/how-many-eas-failed-in-zhigh-risk-high-reward-projects

Wave's first attempt to build a mobile money system was in Ethiopia. I joined them to help with it, and was laid off when it failed.

(They've more recently been successful in Senegal and elsewhere; it's just the initial Ethiopia project that failed.)

If I was smarter and more careful, I probably could've been able to figure out that this wasn't high impact even ex ante, but at the time I thought that running a large utilitarian memes page could've been pretty high impact. 

I think it is fair to call EA Ventures, the Pareto Fellowship, and EA Grants failed projects, and I think each had high-EV. I’ve written evaluations of each of them here. I also discuss the EA Librarian project (though I don’t conduct a full evaluation) which also seems to have failed.

I added a response to the other post.