The biggie I notice is how negative thinking will close doors and opportunities to you. I can think of a couple of instances where I was trying to convey some opportunity or deal, and someone flipped out and reacted very irrationally. When you convey an opportunity and someone flips out on you, i.e. “What, you don’t think I can afford x? I have to settle for y?!?” or the opposite, “What am I? Made of Money!?!” you back off and don’t try to help. A simple, “I thought about that and it’s not for me,” or “I don’t think that is going to work for me,” will do. It means next time an opportunity comes along, I know who not to mention it to. Their loss!