In fast conditions market makers widen or pull quotes, queues churn, and the latency between your screen and the matching-engine stops being trivial. Some venues formally flag the state; in practice you recognise it by quote flicker and by fills that bear no relation to the last print.
Every order type behaves worse. Market orders slip, stops trigger and fill far away, and limit orders that would normally fill simply do not.
Example: a rate decision drops and a futures contract moves 22 ticks in 400 milliseconds. You click a bid at 5,002.00; by the time your order reaches the engine the bid is 4,996.75 and you sell there. That 21-point difference on one contract at $50 a point is $1,050 of slippage from a single click.
Related: latency, quote-fade, stop-order-slippage, market-order-collar