The only issue I've ever heard was one model of a pilots display (so those tv things they use to display pretty much everything to them) would black out in the prescence of a WIFI signal, however, the frequency of the wifi wasnt a normal one, and there was this argument that yes its normal but they cant control all the different hardware some of which may use it. Yes, this doesnt make much sense but yeah. This one time on a flight I was on, on approach the captain said someone had their phone on and this was interfering with their systems, no idea if its true though (i mean, he said it, but was he lying i dont know)