I think I've encountered this type of person. On physics stackexchange there was a guy who would write with great confidence about all kinds of difficult topics

I think I've encountered this type of person. On physics stackexchange there was a guy who would write with great confidence about all kinds of difficult topics, like string theory. When he wrote about my area of expertise, most of it made sense, although he was sometimes vauge and ambiguous. From his interactions with others I would say his knowledge was genuine there too. But every now and then he would make grandiose claims about fringe topics (cold fusion) or really weird political remarks. And when criticized he would snap and insult people. I think he eventually got a lifetime ban on stackexchange. I never figured out if he was a genius or crazy or both. The guy didn't have another online presence as far as I could tell.