Bill Cosby is one of the richest men in show business now of course, but in the '70s he was a total mess (broke and deeply in debt) due to bad investments, unpaid taxes, and a thieving manager.* He talked in one interview about how he was world famous, had a hit comedy record, could dial up half of the Hollywood A-list by memory, but he wasn't able to buy one of his kids a birthday present because he had no cash and all of his cards were not just declined but seized, and how he had to borrow the money from a fan who was in the toy store (who he said he still sends gifts to on his daughter's birthday). His advice to all up-n'-comers is "NEVER EVER EVER give anybody but your wife the power to sign your checks, and only let your wife do it if you've been married a while and have kids and you 100% trust her" (i.e. not some newlywed trophy wife or Sharon-Stone-from-Casino character). This would seem a no brainer, but apparently there are people who allow their managers to just have full access to everything they make.
Sadly, many celebrities with beaucoup megabucks never learn these lessons.