On a larger scale, a retcon can affect initially one person but through the course of that character's interaction with the rest of the MUX, affect others. Hence, retcons are taken very seriously and we as a whole wish to avoid them whenever possible. (No, this doesn't mean you can't retcon a pose as discussed above... this means we don't want to see events come to pass where weeks of roleplay have to be unwritten for one reason or another.)

There are several things which could cause a retcon, the predominant cause by far is likely to be ignorance. Read what people pose, consider it carefully. Do not pose for other people, or incorporate their reactions to your poses in your own unless you have permission and have discussed the reaction.