A wise person once explained to me that in dealing with the multiple shades of gray that real-life usually presents, we sometimes have to cross a line to know that it's there. CrossingTheLine is the process of discovering a previously unconscious barrier, limit, or line. It can be a very helpful process, and is an important part of maturity - in both an individual and a community.

The recent intense unpleasantness in the wiki may be a good example, at multiple levels, of CrossingTheLine.