The only sure way to get banned/suspended on the spot is too insult someone. A really stupid post/topic or a mild insult will give you a probation. A history of slightly stupid (but not stupid enough for a probation on the spot) will add up resulting in a suspension, usually for a week. The best example of this would be MaxTheReaper who was suspended for a week for a post that would not have gotten someone with a clean posting history in any trouble. The line changes for people depending on their past histories. Some people can move further away from the line for being great posters, some people move closer for not being so great. Everything changes and the line for one poster may vary for another.
There is no set line. Not only are many posts overlooked (due to not enough reporting) so one can get away with some subtle stuff. Also the line seems to vary between moderators, only slightly, but I do think that there is variation.
Also, this is what the motherfucking guidelines are for. They pretty much tell you were the line is. Maybe people should read them?