If you unexpectedly find a problem solving itself almost effortlessly, and you can’t quite see why, you should try to analyse your solution more sceptically. Most of the time, the process for solving a major problem is a lot more complex and time-consuming.

I would add that if you unexpectedly find a problem being extremely hard to solve, you should react the same way. Devote a little time to trying to solve it under the assumption that a simple solution exists. It works surprisingly often (i.e., more often than never).