Fermat's Last Theorem
Remember when Andrew Wiles finally proved Fermat's Last Theorem? The final proof turned out to be just 'a bit' (LOL) more difficult than had originally been imagined by Fermat. Orders of magnitude more complicated.
Self-Driving Cars are probably going to be quite similar. When they finally get one actually finished, one that successfully stays out of the headlines and avoids contributing to 'interesting' tragedies, they'll look back over the intervening decade(s) and then laugh at the vast ratio between their final fully debugged system, and the 2016-era trivial kits that some had imagined would be sufficient.