1. Applying too strong force on her clitoris
Clitoris is the only organ made only for sexual pleasure. Sorry, boys! However, being made for only one goal means also being very sensitive. Never use intense force with your tongue, teeth, fingers, sex toys and other stuff on her clitoris.
2. Making too fast movements of your tongue
This is a big no-no. Not only can it hurt, but you are also giving her no chance to concentrate on her way to the orgasm. This is not a pleasurable activity as you may got if after having seen in porn movies, but can be rather considered as a torture technique if the pressure is too strong. However, if you make your tongue nice and soft and the touch fast but gentle, this could be a source of unbelievable pleasure.
3. Making sad faces when she asks you to give her oral pleasure
Doing cunnilingus is not your duty, errand or chore. Even if you think it is, never show it to your girlfriend. Just imagine her making the long face when you as her to give you oral pleasure. Even if you don't like it, fake it that you do.
4. Having bad comments about the smell
The smell of vaginal discharge depends on health status of the woman, her hormonal status, period of menstrual cycle, some chronic or acute diseases, woman's diet she is on, medicines taken, amount of drank water and so on. If the smell is pathologically bad, she should visit her doctor, and you should suggest something like that. However, if the smell is just strong, but your girlfriend is perfectly healthy, simple dietary changes could make the trick. More fruit, less meat and there you go.
5. Having bad comments about the taste
Ok, we all know that it's not anything close to sweet cheesecake or refreshing ice cream, but then again it's not that bad! Or usually it's not that bad. You can suggest a checkup with her doctor, just to make sure that she is STDs negative and to check her hormonal status. If everything seems fine, some changes in her diet may work wonders for both of you. Pineapple is your next best friend. It has been said that after eating pineapple, woman's vaginal juices taste so sexy and delicious that you cannot stop licking. Give it a try.
6. Having bad comments about her pubic hair
You expected a hairless goddess, but Cousin Its relative is staring at you expecting a kiss? Or, you may have had dreams about this woman, and now, when she took her clothes off, she looks more like a prepuberty girl, with not a single hair down there?
For the first scenario, things are pretty easy. Tell her that you find it a bit hard to fight against all that hair, and that you would really love to lick her skin and every inch of her pussy. It's hard to resist an invitation like that.
For the second scenario is a bit different. It's easy to shave but growing hair takes time. Tell her what kind of haircut down there is your favorite, and don't forget to state that if she does it, you going to make it up to her. And then really do it.
7. Penetrating a girl that hates penetration
This could be a recipe for total disaster. Not only she may hate the penetration, but she could also be a virgin trying to keep her hymen. Never put your finger in her vagina before asking if she likes it.
8. Not penetrating a girl that loves to be penetrated
Some girls just love to feel you deep inside their... khm... souls. Come what may, for some girls just a tongue on the clit is not enough. In one part of my life I was totally addicted to G-spot massage and wanted it all the time. Cunnilingus was somehow close related to it, so I insisted on it every single time. Fortunately, my then-partners were full of understanding.
9. Making one or two moves all the time
This would be like reading one book that has more than 1000 pages and only 2 sentences repeating in all those pages. Would you buy it? Well, neither would she. Go, read the "Learn2lick" book and you will get plenty of ideas on different techniques for cunnilingus.
10. Changing pace so she cannot concentrate and achieve her orgasm
I hate this one. And if you try to recall from your memory all the girls trying to give you hand job and as soon as you are really close to cum, the rhythm is slower or faster than you need. And then again and again and again. Remember? Ok, then remember not to do it to her. You are much more skilled than that high school girl you've just thought of after all those years, aren't you? Remember that guys need faster and stronger pressure when they are about to cum, and girls needs the same pace so they can achieve orgasm. When you know she is very close, just keep on doing what you were doing and expect to see the magic doing it's trick!