From my experience, the word "flagrant" is most commonly used for a certain type of highly offensive or outrageous behaviour. So when you are asking about the use "for human[s]" maybe you were talking about somebody's immoral behaviour/ attitude? In that case, it would be fine to use this word. Your other example is fine too, at least I have seen the combination "flagrant error" before (on LEO). This webstie also gives you more sample sentencenes: http://sentence.yourdictionary.com/flagrant

Yes, you can use flagrant for other nouns, such as "a flagrant fault" or "a flagrant insult" or "a flagrant oversight."

Yes, in the sense that it is extremely obvious.