If one has not invested with Bernard Maddoff that one can get a cheap education. If one has, then the education is possible but it is more expensive.
Fortunately I did not invest with Maddoff, never had the opportunity.
The first lesson is - "If its too good to be true, then it probably is not true." Madoff consistently made about positive returns and returns that were above market. So when the market was down he was up, and when it was up, he was up more. No one can do that year after year.
The second is - "Don't put all your eggs in one basket." Granted some people have made fortunes without following this. However they knew exactly what they were doing. Maddoff investors really did not know what he was doing. Those that trusted him with all of the net worth are now in very bad shape.
These are not rocket science but they are worthy of your attention.