One leg shorter than the other
 

You could tell this was coming, right?

But a worthy topic none the less. The good news is when someone is riding, its hard to tell...especially with high cadence riders.
It becomes apparent however when they get off the bike and fall down.

The funny thing is that it only happens if one leg is shorter than the other. Those who have one leg longer than the other can walk just fine.

Actually, I found out that one of mine is significantly shorter (from a biking standpoint) during a pro fitting a few weeks ago. I had no idea.....but had been told by others that my running gait was weird. The fitter suggested changing over to road shoes and clips so he could add a platform to one foot. Unfortunately, I'm on mountain shoes and SPDs for my road bikes so it'll have to wait.

if you start or are experiencing any pain switch immediately

letting the problem get worse will result in longer recovery and less biking

for the sake of some new pedals and shoes... it entirely worth it considering what medical costs can arise if you need treatment