[theorique]

anyone placing a limit order should know that they might get a vastly different price than the one they expected

If you place a limit order at (e.g.) $100, your order should be filled at or below $100 - no exceptions. The order will stay around until it is either filled, manually cancelled, or expires (at end of day or at a prescribed time).

A market order can be filled at an arbitrary price because you are communicating that you are willing to cross the bid-ask spread and meet the market price, even if it's moving rapidly.

[grey-area]

Sorry this wasn't very clear. I meant that you might not get what you expected (though it will conform to the rule you set, if it completes). Stocks can be very volatile and a limit order only controls movement one way, so it doesn't protect you from (say) a huge drop in stock price just after your order.

[uptown]

What does what happens after your order is filled have to do with your limit order execution price?

If you want protection from price decreases after a buy, also put in a stop order (which will turn into a market order) or a stop limit order (which ensures execution at the specified limit price) but which may not execute if the price movement is highly volatile.

[theorique]

That makes sense.

You're absolutely right that there's nothing to protect you from downside if your (long) limit order price gets hit prior to a major nose dive in the price.

(Short sale equivalent: if your short limit order gets hit prior to a major rise in the price)