Does an expert advisor get control after every quote change

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Is an expert advisor guaranteed to get control after every bid/ask change or can situations occur where that does not happen. One possibility is that by the time I attempt to execute at a given price, the rates have changed and by the time RefreshRates has executed the price is gone. These are market orders (buy/sell stops) with slippage set high enough to hit the quoted price.

Has anyone else experienced this and if so, any idea how common it is?

Scott