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There is more than one exchange. If you see a Bid of 10$ come up on the NYSE you can put in a bid of 9.99 on another exchange and if that order gets filled you can then turn around and try to match that bid of $10 on the NYSE if you are fast enough.

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The bid of 9.99 will NOT get filled before the bid of 10, except under very special circumstances, even on separate exchanges. The exchange with the lower bid is required to route the trade to the exchange with the higher bid as a result of RegNMS.

http://en.wikipedia.org/wiki/Regulation_NMS

The only time this rule fails to apply is under extremely high latency scenarios. For example, last thurs when nasdaq left the machines on but NYSE switched to human matching, RegNMS was suspended.

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