While I’m not sure if this quite counts as “illegal”, Set 70706 Crater Creeper has always stuck out to me for requiring such blatant, yet also extremely minor, bending of parts:
In order for the jaws to cross each other like that, the blades must bend out of perfect tolerance ever so slightly.
I get that it’s not a big deal given how thin and flexible those blade pieces are (heck, some of them were probably more deformed straight out of the box), but it’s interesting that Lego would have such a necessary bending of pieces in such a recent set.