Was there a design flaw in the fuel tanks of the MiG-15/17/19/21?
I found a reference to exploding fuel tanks on Mig-21's caused by overheating engines.

 "it had an unfortunate tendency to explode in mid-air. The problem was finally traced to a fuel tank placed under the engines that could cook up when the engines overheated."


  Like its forebear, the MiG-17 inherited the major design deficit which
  could cause the fuel tanks develop to under-pressure if more than half
  the fuel had been used, which could easily lead to tank implosions,
  crushing the main fuselage of the aircraft mid-fight, with almost
  always fatal results.


Tank implosion implies negative tank pressure. Under pressure or negative pressure is caused by improper fuel tank venting.

High pressure pumps sucking fuel out of an improperly vented tank can quickly develop negative pressure which can cause a tank collapse. If the tank was integral to the fuselage it could also cause an airframe failure. A collapsed tank could also leak fuel which could lead to catastrophic failure if the fuel found an ignition source.