A red flag to me is someone who cant talk about their feelings honestly. No matter what the subject is, they should be able to talk about how they feel even if you disagree with each other. Sometimes you will never agree but at least you can have a discussion about a touchy subject with love and caring for each others feelings.
I totally agree: someone who says one thing and does another. Someone who isnt honest about little things in their life, will not be able to be honest about the larger parts of life.
The biggest red flag is your gut feeling when starting a relationship with someone new. If you have a nagging feeling that something is off but continue anyway. Never doubt your instincts for other person's real intent. This has protected and saved me more than once from pain.