The classic forced grinning facies of lockjaw a.k.a. tetanus. Tetanus toxin causes muscle spasm. This affects all the muscles of facial expression at about the same time. When muscles that affect the same thing spasm at the same time, the stronger ones win out, so this condition is sort of like a demonstration of the strongest muscles of facial expression.

This condition is highly painful and is a medical emergency as it is a good suggestion that the respiratory muscles may spasm uselessly in the near future, causing respiratory failure.

sample photo at http://www.immunize.org/images/ca.d/ipcd1861/img0005.htm