Instant execution gives price certainty at the cost of fill certainty. In quiet conditions it fills normally; in fast markets it produces a chain of requotes and you may not get in or out at all.
Some platforms let you attach a deviation allowance, which blurs the line between the two models by permitting a small amount of slippage before requoting. Check which model an account uses before trading news, since it changes the risk profile of every stop.
Example: during a rate decision a trader tries six times to close a losing position under instant execution, is requoted each time, and finally exits 34 pips worse than the first attempt.
Related: market-execution, requote, slippage-tolerance, spread-widening