Just within the last few weeks alone, I've experienced that over a dozen times.
Starting a one-year epic this week, with the goal of creating a product that brings in enough revenue to keep it afloat, it's amazing how many things I thought would be easy, but weren't, and how many more things I thought would be way too impossible to accomplish - but, totally the opposite.
An example was the authentication system I just created. Secure, with proper validation (using RegEx and other methods), properly modelled using roles and groups that went right into the refactoring of the app, then applying email validation, which then rolled into understanding how that worked as well as tightening up the processes involved, along with paying attention and minimizing the time it would take away from the user.
And, that's about a quarter of what needed to be understood and completed before Monday.