This is not exactly a sperm donor situation but my dad dated a women a few times after he and my mom divorced. This lady said she couldn't have kids and therefore they thought they didn't need to use protection. Well, said lady ended up getting pregnant but never told him and they never saw each other again.

12 years later, he gets a call about his daughter. My step-mom handled it very well but I was stunned. We wondered if this lady was lying but when she showed up with her daughter, there was no denying it.

I do think it's great that people donate so others that can't have kids biologically can have a child one way or another.