Buddha's injured leg

This is an English summary of my earlier post, for the benefit of those who cannot read Chinese.
This is a story about Buddha, which is meant to proof that the Law of Cause and Effect is the fairest and most impartial in the Universe. It is so powerful, that even Buddha is subject to it.
One day, Buddha injured his leg. His disciples were shocked. Buddha has already attain Enlightenment, why would he be injured? Buddha explained that in his past life, he injured the foot of his friend. This is his retribution.
When I read this story, I felt that Buddha was pulling a fast one. Having told everyone he has attained Enlightenment and became a Buddha, but now he was hurt, like an ordinary guy. So he came up with a story about some incidents in his past life, which nobody can be prove whether it is true or not. Since he is Buddha, all his disciples accept what he said by faith.