That's actually a very good question.
Bad
slippage on exits can happen during extreme
volatility or low
liquidity times with all brokers, but is more likely to happen with
bucket shops than ECNs. The worst brokers will use this to deliberately hold your profitable position open once the market starts to trend against you.
All brokers have disclaimers like this, but some have worse records than others. Definitely trade with the smallest amounts possible until you learn the limitations of the broker.