Tin forms whiskers, small hairlike growths on the metal that can cause short circuits. We've already lost a satellite to this phenomenon, although I would hope it's not a problem anymore with current metallurgical techniques
The problem was ultimately found to be a combination of pedals being pressed by floor mats, pedals sticking on their own, and drivers pressing the wrong pedal.
That's not to say acquiring non-RoHS components isn't made more difficult by RoHS.
http://www.militaryaerospace.com/articles/print/volume-20/is...