One of Mongo's fatal problems is that the database has been known to say it has fulfilled its durability promises, but that's only sending a packet to the kerne

One of Mongo's fatal problems is that the database has been known to say it has fulfilled its durability promises, but that's only sending a packet to the kernel, not confirmed received by others. Same with writing to disk. So a crash can lose more data than originally thought. Other things like continually pointing to each other, not figuring out a master for hours has made some people tear their hair out, despite the documentation saying it should behave in a sane way.