I am a software developer who recently left for a new employer. In my last few weeks I met with a lot of people and did knowledge transfers to help with projects I had worked on that may need work in the future. You can only go over so many things and ultimately some things will be overlooked.
Long story short, I was contacted by someone who still works there asking for guidance on an application that I had been involved in. I know I've been in a similar position before where the subject matter expert had left (albeit with no knowledge transfers) and I got a project dumped in my lap. I thought then 'boy, it would be nice if I could just contact them to get a start', but ultimately didn't do that and struggled through it because I thought it would be unprofessional. Is it unprofessional to contact someone who left the company for help?
From the other side of the spectrum, is there anything wrong with answering their questions?