It's kind of sad that we'd need an example to show that it really could happen in every single specific case. It should be common knowledge by now that competent devs screw things up all the time.
For example, I'm sure the MongoDB devs are extremely competent. But all the same, having a database management system default to letting writes fail silently is a pretty spectacular screw-up. I can't really blame other competent devs for taking it for granted that a DBMS wouldn't do something like that.