Yes - my former boss and I had a really great working relationship and personal chemistry (perhaps the latter fed into the former). Chemistry aside, he was a fantastic boss in everyone's eyes and a wonderful human being. We kept things professional while we worked together, but when he left the job, we kept in touch and eventually ended up hooking up. Unfortunately, things got really awkward between us because I misunderstood and thought he wanted something more serious than FWB (and I was actually okay with FWB since I just got out of a long relationship, but several communication issues led to an irreparably awkward dynamic).

I do regret that he's no longer a mentor in my life, but I would've regretted the "What if?" possibility way more if we hadn't just gone for it. It's MUCH easier to find mentors than it is to find someone you have great chemistry with, and it's hard to suppress chemistry anyways. If you want to try pursuing something, you can always try to search out a different mentor figure to fulfill this one's shoes. If you want to keep him/her as a mentor, then perhaps try to distance yourself for a bit until your feelings subside.