Unbearable fright.

From early times there are stories of the trials and tribulations involved in writing plaques for high palace buildings. For instance, in the Wei dynasty when the Ling Yun T'ai—a recreation pavilion "as high as the clouds"—was being built, Emperor Ming had the famous calligrapher Wei Tan write the plaque inscription. Wei had a platform built at a height of seventy-five meters and climbing to it, wrote the inscription. When he got down his hair had turned white from unbearable fright. He admonished his children not to become calligraphers.

—NAKATA Yujiro, The Art of Japanese Calligraphy
which I highly recommend