They didn’t go into full-on tank mode until after they lucked into Durant (or, the #2 pick, at least). That’s when they decided to gut the team, and picked up the #5 (Jeff Green). Clay Bennett knew he could tank and surround Durant with guaranteed Top 5 picks over the next couple years, build something good around one superstar, and just print money as the only game in town. Problem is, he landed three stars (super-max players), they got too good, too fast, and then it was like “holy crap, we actually have to pay these guys!”
So, the lesson really is: tank, but don’t accidentally draft three superstars unless you’re in a big market.