If you have multiple clients and everyone knows what's going on and everyone's okay with it then obviously we're in a completely different zone. I've had contractors that I was paying and they asked politely if they could spend a little bit of time working on another customers immediate problem and I gave them the okay to do that. It's much like an open relationship: it's only cheating if you're breaking the rules.

I would say, be very careful if you are in a full-time position and you start a side business. Depending what it is, you could end up in a situation where your employer ends up owning your work. When I wrote my book, I made absolutely sure not to do it on company time or to use company equipment, because if you're not careful they can end up owning the copyright to a work you've put a lot of time in. A little side thing like that probably wouldn't be so bad though, particularly if it's something where you're your own boss and you can put it all down the moment that you need to if the quiet moment passes and you need to get back to work.