In "The Minister's Black Veil," what happens to Mr. Hooper when he sees himself in the mirror?
On the day that Mr. Hooper begins to wear the black veil, he first delivers his sermon, and the topic is "secret sin, and those sad mysteries which we hide from our nearest and dearest, and would fain conceal from our own consciousness, even forgetting that the Omniscient can detect them." Such a subject allows readers to begin to understand that the veil is a symbol of this attempt to hide our true sinful natures; Mr. Hooper wears it as if to admit that he has such a nature, as we all do.
After the sermon, Mr. Hooper presides at a funeral, where the solemnity and darkness of his veil seems appropriate, but the wedding he officiates later on is affected in quite a different way. Here, the bride is overcome by a "deathlike paleness" when she sees her minister, and the groom's hand is "tremulous" when he holds his beloved's. When Mr. Hooper raises a glass to toast the new couple, he "catch[es] a glimpse of his figure in the looking-glass, [and] the black veil involved his own spirit in the horror with which it overwhelmed all others." Just as Mr. Hooper's "hearers quaked" when he preached upon the subject of secret sin, the minister himself seems now just as fearful of his own aspect. Seeing his own reflection, he is reminded of both his own secret sinfulness as well as the secret sinfulness of everyone around him, and, given that (as the narrator said earlier) we all desire to conceal this sinfulness from ourselves and everyone else, the visual reminder of his sinfulness -- in the form of the black veil over his face -- alarms Mr. Hooper enough that he actually drops his wine glass and rushes from the building.
Mr. Hooper has chosen to wear a veil, for reasons that are not explained to us. Following a funeral, where it is suggested that he knew the girl who had died, and that this was somehow the reason for his wearing of the veil, Mr. Hooper is officiating a wedding. Following the ceremony, Mr. Hooper wishes the new couple good luck, but he sees his reflection in a glass of wine as he raises it to his lips to drink; when he sees himself, and the veil, reflected, he is momentarily overcome with the same sense of horror that everyone else feels because of his appearance. He shudders, his skin pales, he drops the glass, and runs away. From that time on, he avoids glasses, mirror, or standing water, so that he doesn't accidentally see his reflection.