Mistakes that make you unlikeable

From Penelope Trunk's article Making Yourself More Likable at Work:

Being quiet because you're insecure
If you have nothing that you want to say, then you're probably not likable because you have nothing to offer. But if you do have things to say but don't say them, then you're not likable only because you're so insecure that you believe you'll sound stupid when you talk.

Being too smug
...as in not asking for help or not revealing that you've had help along the way. To show no gratitude or no need for others is to alienate yourself.

Not seeing people for who they are
If you treat people who are powerful well and people who have no power poorly then you aren't seeing the whole person. Power structures don't define a person; they define a person's clout. Treat everyone with respect or you won't deserve it yourself.

Not taking responsibility
If you blame people for your problems, people aren't going to like you--even if they're not among the people you blame.

Hiding from objective feedback
You can get it from therapists, co-workers, teachers, and coaches, but you have to seek it out. And if you don't, then you probably don't have a good sense of your least likable qualities. So you don't have the knowledge to make yourself likable.