They do not ask enough questions. Or do not admit to not understanding something.
In some cases its a cultural issue. For example working with some (not all) developers in Asia, its culturally negative to admit you don't know something. I've found ways to work around this and bring some really talented people into good heights.
But for western/anglosphere developers, it's the kiss of death.
I've found that Eastern European developers generally believe they are the best (even when they aren't). For instance, a young developer I eventually hired asked me if I knew something about a topic (I have for decades) in a very condescending way. This happened multiple times from different Eastern European developers. Once you understand that cultural imperative, you can have really great coding relationships with those developers.
#2 sign of a bad developer?
They don't do enough reading. I mean they don't keep up with the industry, don't follow HN, don't keep up with the latest news on the tech stack they use, etc.