I believe that one should be more scared while driving a car on highway than leaving your monitor after placing the stop order on forex.
I guess the majority of 13,000+ members of our Forum are trading
demo money or small accounts. When you have capital of $300 invested in Forex you have nothing to diversify.
Tiny 10k and "standard" 100k lots usually are not cleared thru the market, so it is the computer who executes orders on them. And there is no problem for him(it) to stay awaken 24H and make slippage=0.
Forex spot market is not like stocks or futures where a trader is facing price gaps all the time.
If your are trading majors - slippage is minimal.
If you are in the middle league - tens of thousands account size - then you can press your broker in case of big slippage, to post about him on forums, report to regulators etc. and in most cases he'll give you your slippage back.
For the big league where you have tens of millions open position you have the last resort to sue your broker for damages and probably will win. It is a good idea to have at least two brokers/banks for big money. And stay ready to "some"

slippage.
Anyhow you may call to the treasury of your bank or Reuters to get second opinion about highs/lows, volatility on the market at a specific moment and press your broker.