Once I hit two 4 of a Kinds back to back. The probability of hitting it once is 0.24%. The chances of hitting it back to back are 0.24%2 = 0.0006%, or 1 in 179,000. That's four times less likely than hitting a royal flush. And I would have preferred the royal! (By the way, purists will note that after you've hit a 4 of a Kind, the chances of hitting another four of a kind is just 0.24%, not 0.0006%, but I'm considering the probability of getting two back-to-back before I get the first one.)
Well, the charts on our site are on roughly accurate to within 0.1% of perfect play. So, if you find that game that returns 100.5%, you would be actually earning about 100.4% with our charts - not too bad. Not many charts get much closer than 0.1% because they would have to be extremely long and complicated to explain the subtle differences in rare hands.