Possible causes for a valve to double !!!

Good afternoon.

Friends, I would like to consult what are the causes that can cause an escape or admission valve to bend ???

On the other hand, a mechanic tells me, that after doing a complete service to a camera, where the valve guides and their approaches, and the contact surface with the block, the latter if it is rectified too much, can cause it to cause the engine to be up Turn off because it folds the valves, since the camera was very low, and the pistons are demanded more, so they make more route and come to collide with the valve, is this true ???

Thanks and greetings !!!

The valve will fold if it collides with the piston, that is true, because there is no other way that this happens naturally clear, if there is something inside the camera it may happen, but it is not very likely Tolerance, if the chisronization of the times is not fine the valves will be folded even if the cylinder head and the Blopcke have not ever been rectified.