On Tuesday, August 11, 2015 at 8:13:39 AM UTC-4, Bob Pasker wrote:
I've never flown a contest, but I have a question about people coming up short. If that seems to be a problem, why isn't there an altitude floor for the finish line? --bob

There is, but is can be quite small. Most contests use a finish cylinder with a radius of a mile, with a minimum finish height appropriate for the site, commonly around 500 feet or so.
A few places can still use the lower line finish(bottom about 100 ft AGL)where it is deemed that it can be done with adequate safety.
UH