Discovered that a friend was a donor sibling
Only a few weeks after I posted that I was looking for potential siblings for my daughter (when she was about 8 months old), we were contacted by the mother of a donor sibling through this site. To our big surprise, I already knew both the mother and the child. The mother and I had gotten to know each other during our pregnancy. Our children were born 4 days apart, live 10 miles from each other and have spent time together since they were born. Suddently we understood why we were dealing with the excact same issues with the children at the time, but we would never had guessed that they had the same donor.
I consider it a great gift for my daughter that she will grow up knowing the other child :-).