[ it's so easy to sprain an ankle ... and not to be able to go on.
(please : I chose the "sprained ankle scenario", it's the least painful but handicaping I thought of, don't give me worst examples of what could go wrong .... )]

Your pick of a sprained ankle scenario is an excellent one. However, just be aware that a sprained ankle can be extremely painful and debilitating, even more so then many fractures. Pete