When someone hits a new spot, for instance a freeway shot or rooftop there is a certain amount of supposed respect to be garnered. When another person, after seeing this new spot done, goes and puts up something (piece, throwup, whatever) next to the first one, it is considered the act of sidebusting. Its a sticky subject, some writers care, others dont mind at all. One of the supposed reasons for taking offense to sidebusting is that it will heat up a spot, making it more likely that the original thing painted there will be buffed sooner. Choosing a good spot and having it run a long time thus generating a lot of exposure is something which is highly sought after. Another alleged reason is that that people besides the first hitting that spot are trying to leech off the fame and ingenuity of the originator. In the end this often leads to beef which will escalate itself (as most conflict tends to) into crossing out, fights, and a warm fuzzy general hostility.