spam in the ban section, posting offensive/adult material, racism/insults etc.
If you see someone muted or warned, there are 2 highly likely reasons and 1 uncommon one.
Tied for top 2 are posting witty/stupid things alike in the ban threads and saying friend. Although a very sure way to get a mute is by saying friend a lot in the ban section.
Third is when a bunch of people are organized into reporting one slightly rule-breaking post in order to fuck with someone. That one's hard to pull off.
Naturally, I don't speak from experience, but only observation :^)