Introduction
In the first part of this post I've explained some of the details and underlying reasons of bug 6918210. The most important part of the bug is that it can only be hit if many row migrations happen during a single transaction. However, having excessive row migrations is usually a sign of poor design, so this point probably can't be stressed enough:
If you don't have excessive row migrations the bug can not become significant
Of course, there might be cases where you think you actually have a sound design but due to lack of information about the internal workings it might not be obvious that excessive row migrations could be caused by certain activities.