Well my guess is that the deraillor ran into the spoke protector(what you called a plate). It is also referred to as a "dork disk" :-). In this case it did its job and stopped the spokes from grabbing the deraillor :-).
There is a limit screw that should prevent this from happening. So to avoid that until you can get the bike you buy in for a tune up, avoid shifting into the gear closest to the spokes. The weight of a rider really has no bearing on this adjustment :-).
In the late 1980's so called index shifting took from the older friction shifting, if you can get onto a bike with index shifting it us much nicer, you just click from gear to gear instead of hunting for it by feel.