I ran a forum before this with a really goofy domain name (which was also a mistake). It was the first time I ever ran a forum, I've known about my mistakes for a while but I just felt like writing about them now.
#1 I knew that the combination of a left-leaning admin (me) plus my aggressive debate style was causing conservatives to be disproportionately banned, leading to a left-leaning population (which I did not want). My solution was to deliberately hit the weakest liberals on the forum with trumped up charges to ban them.
#2 I banned a guy because he started every post with "sorry bout that", and this went on for more than 100 posts.
#3 I argued too aggressively with a guy who didn't like my favorite rap songs. I indirectly insulted him, thus leading to an escalating insult contest resulting in his banning.