That IS an excellent thread and explanation. I've been involved with this scenario a couple times and learned what NOT to do! Basically, don't tie in a multi-wire circuit or part of one (any circuit sharing a neutral) while more than 1 leg is still on. Better yet, turn the whole group off. I was forced at the time to work for speed, and not allowed to shut anything down. When the neutral splice came apart (Surprise!) office equipment started leaking smoke and there was alot of screaming and pandemonium.
A friend of mine recently got his service upgraded, and the electrician showed him that his utility neutral had been completely unattached, probably for many years! The current must have been flowing through the waterpipe ground, because they never had any noticeable problems with their power.