One famous, or infamous depending on your view, example of a retcon was when it was revealed that Norman Osborn was the father of Gwen Stacy's babies. When Gwen Stacy went to Europe after her father's death, it was revealed that she had been romanced by Norman in a moment of weakness. She gave birth to twins and wanted Peter to help raise them, but Norman found out and it was this that drove him to kidnap and kill her.
Sometimes the retcon will come under much fan scrutiny and more and more loopholes will be discovered in the retcon. That is one of the dangers with changing comic book history.