“A mail carrier was walking on grass that was spongy and wet when both of his feet sank in. He fell forward and then fell back with both his feet still stuck in the mud. The employee was unable to bend both knees and was taken to the hospital and admitted for evaluation and treatment. Ligaments and tendons in both knees were detached. At the time the incident was reported, it was not clear whether this caused the fall or this was caused by the fall.”