The biggest surprise for me was to grow up and learn to know what I didn't know. We all get institutionalized to a certain degree. And through our institutionalization, we get older and take so many things as a given, things we've believed our whole lives, but some reach the point in which you have to reevaluate everything. And life is never the same afterwards.
Some grew up being racist their whole lives and grew to realize how foolish and hateful it was. Some grew up in a religion or cult that they've finally learned to reconsider. For others, they've grown out of their political affiliations. Some finally fell in love with the right person and some finally fell out of love with the wrong person.
Whatever it is, some people finally evolve and grow to a point where their eyes have opened wider and see things from a better, clearer perspective. Whether you like it or not you know it's the truth. This has happened to me, and this is by far the biggest "surprise" for me: unlearning what I thought I knew.