One sobering aspect of the faeries is their inexorable sense of vengeance. If a human offends them in some way, they will never forget this insult and will never fail to extract amends or punishment, however long it may take (in human terms).
One example comes from Dun Osdale on Skye. A man saw the faery mound there open and the inhabitants outside dancing. He spied on the festivities for a while, but then sneezed- and was exposed and caught by the sith folk. They dragged him inside the mound, but rather than chastising him for spying, he was (apparently) welcomed and rewarded with a sumptuous banquet. The man, though, was too canny to consume faery food and drink, knowing that he would have become trapped in the knoll. When offered a goblet of wine, he poured out the contents- and made off with the cup. The thief managed to get across the Osdale River ahead of his pursuers and, having crossed running water, was apparently safe.
When the man got home with the faery cup, his mother decided that it was wise to put a charm on him to protect him from any future attempts by the faeries to take revenge. She did not, however, enchant the stolen cup. It retained its faery mystique, or glamour, and the fatal quality that whoever saw it had to take possession of it. As a result, her son was murdered by another human who wanted to have the precious cup for himself, thereby acting as an instrument of faery judgment. Once your fate has been decreed by the fae, it cannot be escaped.
Another case from Skye indicates how faery vengeance can be cumulative and inexorable. A man took stones from the faery knoll of Dun Gharsainn, removing them at night because he knew that his neighbours would complain if they realised that he was behaving so selfishly as to do something that could bring down the faery wrath on the whole community, rather than just him alone. As it happened, the faeries were absent from their home when the damage was inflicted. This encouraged the man to take more building materials, but on his third visit a light shone out of the mound and a voice warned him that revenge would be taken. The faeries abandoned their ruined dwelling in great distress, but their departure didn’t spare the stone thief. First his horse died, then his cows, then his crops failed. After his fishing boat sank as well, he emigrated. We might assume this saved him, but we shouldn’t count on it. A Jersey man who vandalised a dolmen in a similar way was progressively deprived of all his wealth, possessions and family before being drowned as he tried to sail away to a new life on the British mainland.
Finally, I have told previously the story of the curse of Pantannas in North Wales. A man who had outraged the tylwyth teg by ploughing up the place where they danced was harassed by them as a result. He tried replacing the sward to appease them, but this merely postponed rather than averted their sanction, which fell upon descendants in his family generations later. Faery wrath is implacable and- it would seem- indiscriminate, in that the sins of the parents can be visited upon their distant offspring.