Will it work?

This is related to a post in the road bike forum, but this is probably a more appropriate place to post it.

I am thinking of putting 10 speed Ultegra brifters on a road bike of mine that runs an 8 speed cassette and index shifters. Any reason not to do this?

If i shift 9 times, will the derailleur yank the chain off the gears, or will it just reach it's max cable tension and stop shifting once it reaches its limits?