haircut advice

I got my haircut and I noticed the following day after washing it (and it was straight) that one side of my hair was longer than the other. It looks pretty lopsided. I texted my hairdresser and asked if it was intentional and she said that she probably forgot to cut one side. Now I've been going to her for at least 2 years and never had a problem before. I paid $75 (for the haircut). I'm going in this week to have her look at it and fix it, but my question is should I tip her again? She made a mistake. I feel like $75 is pretty steep for a haircut, so I guess I feel like I shouldn't have to tip for what I've already spent. However I feel obligated to tip in order to show appreciation and keep the relationship. I don't know what I should or shouldn't do. I'm always nervous about how she may feel about my tip like it's not adequate. Anyways just wanted to get some feedback from other ladies.