Assume a hypothetical situation where a researcher was able to develop a computer capable of 2,000 PH/s (the current network hashrate). During his first 2 weeks of mining, he would strike it rich as he would win 50% of block generations with 5 minutes per block.
After 2 weeks, the difficulty target would readjust, and this miner would no longer have an advantage.
However, for this next 2 week period, the miner sits out, waits 2 more weeks, and allows the network to re-adjust back to a higher target, at which point he re-enters and sweeps the rewards once more.
He repeats this cycle in perpetuity.
A solo miner with 2,000 PH/s seems infeasible, but what if a powerful mining pool were to pull this stunt — withdraw from mining for 2 weeks, allow for the difficulty target to re-adjust, and re-enter with a much easier difficulty?