Does nobody understand statistics and probabilities here?
Would you say a roulette wheel was rigged if it came up black 10 times in a row? Even worse, would you bet heavily on red for the next spin?
Every single block generation is a separate occurrence, estimates of total hashing speed are based on a very simple formula for average time to solve a block over X number of blocks vs current difficulty. A lucky string or two of quickly solved blocks will make it look like hashing power went through the roof. An unlucky string of blocks taking an hour to solve would make it look like hashing power dropped off considerably.
it seems people take well to casino examples.. here is one about the martingale betting strategy and roulette. taken from wikipedia.
The odds of losing a single spin at roulette are q = 20/38 = 52.6316%. If you play a total of 6 spins, the odds of losing 6 times are q6 = 2.1256%, as stated above. However if you play more and more spins, the odds of losing 6 times in a row begin to increase rapidly.
In 73 spins, there is a 50.3% chance that you will at some point have lost at least 6 spins in a row. (The chance of still being solvent after the first six spins is 0.978744, and the chance of becoming bankrupt at each subsequent spin is (1-0.526316)x0.021256 = 0.010069, where the first term is the chance that you won the (n-6)th spin - if you had lost the (n-6)th spin, you would have become bankrupt on the (n-1)th spin. Thus over 73 spins the probability of remaining solvent is 0.978744 x (1-0.010069)^67 = 0.49683, and thus the chance of becoming bankrupt is 1-0.49683 = 50.3%.)
Similarly, in 150 spins, there is a 77.2% chance that you will lose at least 6 spins in a row at some point.
And in 250 spins, there is a 91.1% chance that you will lose at least 6 spins in a row at some point.
!!?! 
