For every trade I use a stop. I use distance from the entry level to the stop to calculate the amount I am willing to risk per trade.
Let's say if my stop get hit, I'll lose £100. Then price goes to my stop level, but my broker doesn't close my trade at my stop level and let it run into a bigger loss. They eventually close it, but my real loss is now £113 instead of planned £100. That is 13% more than what I was ready to pay for that failed trade. If I risked £1,000 with 13% extra that would be £1,130 etc.
PS It's easy to find out - just check your stop order level and the level it was executed at. If it's worse - you have a slippage
Well if you find that a little hard to believe, you may have a problem