Occasionally but pretty rarely. By the time a trade has become actually risk free, as in "I can instantly buy X here and sell X there and make a profit", someone will have realized that and done the trade when it was almost risk free- the that the price is likely but not guaranteed to go up there- and made the trade first.
Doesn't matter if they're so fast they can't lose on any single trade, there's always a risk of shooting themselves in the foot.