Jessica Allen became a surrogate for a Chinese couple back in 2016 and while pregnant with their child, she became pregnant with another of her own!
Six weeks into the pregnancy, it was found in an ultrasound that there were two babies in there and when it came time to give birth, everyone expected identical twins. Allen delivered the babies via C-section and never actually got to see them in person. However, when Allen was shown a picture she noticed one "twin" was darker than the other and they were not identical.
Eventually a DNA test confirmed the baby was not a biological twin to the implanted embryo and was actually Allen and her husband's biological son!
In an extremely rare turn of events, Allen and her husband say the new surprise baby has completed their family and added joy to their lives. They did say, however, to make sure your contracts are "airtight" when becoming a surrogate! Source: ABC News