I think if you're truly (1) confident about your knowledge or mastery of a particular topic and (2) actually are knowledgeable, you don't have trouble admitting things you don't know.

Here's what I really hate though. When you are around multiple people who are the type to claim knowledge about things they truly have no idea about.

For example in social situations or work situations and you have multiple of these types around. When a conversation comes up and the knowledge of the people involved is put to the test. Maybe your boss is asking who knows how to perform a certain task, or who has experience with certain hardware or software, etc. Or maybe a friend is asking what is the best TV they should buy, etc. There's the people who claim to know about topics XYZ when they really don't, and you, naturally, admit that you don't know. But you might still be the most knowledgeable person about the general topic, and best candidate to offer advice or take on said task. Doesn't matter, you get labeled as the uninformed person while the "idiot" who claims to know everything about XYZ appears to be knowledgable expert. And worst case scenario is when your boss, co-workers, friends, etc. are dumb enough not to catch it.