I think you should be afraid of only knowing one technology/language. You should strive to at least play with one new technology/language per year. "language" c

I think you should be afraid of only knowing one technology/language. You should strive to at least play with one new technology/language per year. "language" can be flexible, like CSS, SQL, HTML. It doesn't have to be things like Go, Erlang, or Haskel. It would be nice to be at a company that encourages that. Even if you just know why people like certain languages, that would be helpful.