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.
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.
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.
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)
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.