Twice in my career. First was a contract to hire position that I had found out from other people in other departments that I was the 6th person they hired for that position. End of the first week, I realized why. The IT manager was not keen on anyone who knew more than she did and got rid of them if they did for various reasons. Terminated my contract after 2 weeks for "creating a hostile work environment." What was I doing? Working in the lab with my headphones on trying to image all the machines she gave me for the techs in the field. Found another job a week and a half later as the sole sysadmin and it provided me the bulk of my current experience.
Second was very recent. Signed up to work for an MSP as they put on a very good show in the interview process. I should have realized that something was wrong when I was forced to sign an agreement that I would work there for a minimum of one year or face a monetary penalty. It was evident what the issue was after a month. Owner was a distrusting, micromanaging asshole who pretends to be a nice guy and concerned for any personal problems you may be having. We butt heads a LOT during the last month there and I let him and his second in command have it verbally after he demanded I write a letter detailing how I will prevent a client from asking I not be sent to their site again because I "took too long." (PC was fighting me left and right about booting from any USB device and the PC did not have a built in CD/DVD drive AND I let them both know what was happening in detail.) A week before I received the formal offer for my current job, he let me go. My only regret was that I didn't get to do it myself when I was ready.
Much happier now with way better benefits and a 15% pay increase with regular bonuses and raises.