I currently have a job at a software company (in the US) in which recent quite a few senior people have left recent for managerial related decisions. The company used to be a great place to work, but it is going downhill. Two of my former co-workers have recommended me for a job at their new company (which they love). I sent in my resume and I immediately received a very eager response to come in for an interview and that my previous co-workers and some former classmates of mine who work there all had very good things to say about me. Honestly, I expect the interview to go quite well and I would not be surprised if they send me a job offer.
My question is, exactly what point during the process is it safe (and appropriate) for me to put in a two-weeks notice to my current employer? After I have a start-date at my new job? After I sign the job offer?