Being teamed up with someone who insists that everything be discussed before it's added to a project. And you do, and it's agreed to add it. Then, they turn around and tell you that you're "doing too much", less than 24 hours later. Claiming it's no longer a "team thing" anymore, it's all about me. While they never lifted a finger to add any content of their own, whatsoever, anywhere.
People who want to control what every aspect of your idea. Thinking that they know better, when you have a clear vision what it should be and how to go about it. Their usual excuse is, "Well, lets think about it", after you've already done the thinking.
People who say they give you a forum, but don't give you all the Admin features you need to build it, the way you want to. Then argue with you, claiming that you make "demands" when they are the ones who put themselves in the position to do what you ask.
Same person says that they'll help you, then disappear for days, while you sit there waiting for them to follow through with their promise of assistance.
Lesson learned:

Don't trust anyone who keeps a bit of the master controls for themselves. You will become a slave to their whims, work ethic, excuses and rants. Run away. Far, far away.
Don't continue with a friendship after you discover that the person is manipulative, argumentative, volitile, bad-mouthing or insulting to your friends, ideas, dreams or values. You will grow to dispise this person.
Don't get chummy with someone, just because they say they like your ideas and want to partner with you. You never know what their real intentions are, and they just might steal them from you, or hinder you from obtainging your goals and dreams. Once you realize this, end the relationship immediately and block any and all communication, however you can.
Be very carefull about what you say when conversing with these people. They will take your words and twist them into meaning something mean and vindictive, then attack you with them when you're not expecting it. These people assume the negative, whenever and wherever possible. The world and the internet are full of these people. Be warned. Never get too close, too fast.