No I haven't heard of that. I give mine a little taste here and there as a treat because they really like it. The yogurt itself actually can be beneficial, I don't eat plain yogurt though so it does have sugar in it which is no better for them than it is for me.
I haven't heard of that either. The only thing I can think of is it were "light" and had asparatime in it. I've always heard not to feed that to them, and never have, but I've still never heard of a tiny bit actually killing a cat. What exactly happened here? Was the cat ill in the first place?