We've seen several flash crashes in the past few years, and I suspect algorithms are some of what's behind these (see this as an example). In a short period of time, an exchange will suddenly see tons of sells and fall dramatically, only to self-correct.
Using an example of the SPY, would it be legal to put in a Good-Til-Cancelled order for $100 a share and make a profit on the flash crash, if it occurs? Note that the assumption here is that it crashes in a short period of time (within a day), not over time (so the order can be re-arranged if the market declines over a period of time).