Since I have been auto trading the Pro Trading system there have been four instances of large slippage on the GBPCAD pair. On two occasions they were positive and the other two were negative. Over all large slippage is -20.3 since late March 2018.

27 March Buy 0.06 Open Slippage +33.6 Close Slippage -1.6 Total +32
27 March Buy 0.07 Open Slippage +31.5 Close Slippage -1.7 Total +29.8
25 April Buy 0.07 Open Slippage -1.4 Close Slippage -33.1 Total -34.5
07 May Sell 0.08 Open Slippage -1.1 Close Slippage -46.5 Total -47.6
If you aim at nothing you will hit it every time.