Forbidden Love and Dating Advice

There are always people in our lives who because of their position or relationships make them a complicated dating prospect. These can be people in authority positions, people we live or work with, or people who've been in past relationships with our friends and family. These potential love interests fall into that gray area, where you're not crossing any ethical or legal boundaries, but there could be uncomfortable ramifications if things go south. I want to know, when it comes to getting romantically involved with these questionable guys, when it is OK to go for it and when are they off-limits?