My sister experienced the following at Mass last Sunday:
An elderly priest, who normally does not celebrate Mass at her parish, presided over the Mass. He was visibly frail, and his hands shook as he held up the bread and wine during the Consecration.
The part that was most troublesome in many ways, however, was the homily. He appeared to be suffering from dementia, and his homily went on for about forty minutes because he kept repeating his ideas and going in circles.
The congregation, in response, was not very gracious. The pianist started playing a instrumental hymn about twenty minutes in. Some older ladies in the back started clapping. One person got up and stood near the pulpit. Although he didn't seem to notice these things, he did eventually conclude his homily and proceed with the rest of the Mass.
This made me wonder: what is the appropriate response, if any, when the priest who is celebrating Mass seems to be incapacitated in some way?