I know of an incidence where a shodan was looking to become a nidan. (This was not my dojo.) Anyway, he was a very aggressive person and when new students showed up he liked to "show-off" and become very hard and aggressive with them. (needless to say many did not return)
Anyway, he approached his sensei and demanded to be made nidan. When the sensei refused, he actually attacked him by trying to hit him in the face. The sensei blocked the attacks and reprimanded him. He then kicked him out of the dojo.
This student was also involved with kendo and several other arts. Word spread and he was kicked out of those dojos for his behavior. Last I heard he had approached the kendo sensei and asked to be allowed to rejoin. The kendo sensei agreed but with the stipulations that he should calm himself and do several hours community service.
It was a disgrace to hear that someone had made it that far and then to actually loose control. I believe that he wasn't really into the philosophy of aikido and that is the real shame.