Unfortunately receivers don't shut down soon enough. When they run out of power to reproduce the requested waveform, they tend to "clip" the peaks of the signal off. This clipping is a very harmful form of distortion, which will eventually damage the speakers
This was my experience with my Denon 2807 which I never got to shut down but did get it to clip.
On the other hand, sometimes you can still push a receiver too hard without it running out of power. Using a speaker with too low of an impedance will cause the output devices to produce more heat than with a higher impedance speaker. If the heat builds up too quickly, the thermal protection of the receiver can kick in, and shut it down. That's not as dangerous to the speakers, as clipping, but the extra thermal stress on the receiver may shorten it's life. Plus it's annoying to have it turn off right when you're starting to rock out.
This was my experience with my Onkyo 3007 when I tested it out. Never clipped but finally shut down after 2 hours at 100-105dB.