Looks like its happened more than once. Also, this perspective doesn't make it look so steep. http://www.nspwny.org/pic-hv avalanche 20021214.jpg
agree with dr jeff, in some cases those "slides" are due to poor technique, which is probably the case with the above. it doesn't look that steep. killington has had slides on vertigo, ovation and double dipper all of which are very steep.