Quote from SimpleTrades:
With maintenance margin of $7.5/1K, it wouldn't take long to wipe out a mini account.
7.5 + 7.5*2 + 7.5*2^2 + 7.5*2^3 + ........
You do the math.
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Not long at all.
Win small - Lose big, what a system to advocate, average down to break even
"the gambler usually wins a small net reward, thus appearing to have a sound strategy. However, the gambler's expected value does indeed remain zero because the small probability that he will suffer a catastrophic loss exactly balances with his expected gain. The likelihood of catastrophic loss may not even be very small. The bet size rises exponentially. This, combined with the fact that strings of consecutive losses actually occur more often than common intuition suggests, >>can bankrupt a gambler quickly<<"
http://en.wikipedia.org/wiki/Martingale_(betting_system)