I encountered once, because the first time I opened my account I never set the price deviation and never provide stable Internet connect that could be sometimes the main cause of slippage.
Not many times, but I still didn't leave my broker because I learned them a lot. They're the one who taught me about Forex market through educational materials without any charges.
The absence of the slippage in my broker makes my trading activities more fruitful. I never encountered slippage during news times or sharp price movements because the platform of my broker is very reliable and they also have a stable servers.