How about 300' agl over a heavily built up residential area in the heart of Toronto? It happened last summer. According to Transport Canada the operator had a ministerial waiver to fly at 600 agl' on this one occasion. However it was obviously closer to 300' - I could see the machine through my second floor window without looking too high. He was flying slowly in a light wind. I don't think he would have much margin if he lost an engine plus the noise level was quite high.