One of the earliest examples is the Greek myth of King Midas. Everything he touched turned to gold. When he tried eating and drinking, he discovered the downside. (This also qualifies as No Good Deed Goes Unpunished and Be Careful What You Wish For, as his golden touch was a wish granted by the god Dionysus as a reward for the hospitality Midas had shown to his friend and tutor Silenus. In Dionysus' defense he did try to warn him what a stupid wish it was.)
Nathaniel Hawthorne's version of the story in A Wonder-Book for Girls and Boys (1852) has Midas embracing his daughter and turning her into a gold statue.
This line was considered so horrific in its implications that it was eventually cut out and replaced by a shorter version that skips straight to the phone gag below. You have to wonder exactly what the thinking was when they decided that portraying an innocent family man with a Fate Worse Than Death was a good way to sell rainbow-themed candy.
He then proceeds to answer his phone, turning it into Skittles, then hits the desk in frustration, with predictable results.
There was also a children's book that referenced this, The Chocolate Touch, by Patrick Skene Catling (the main character's family name was Midas to complete the reference); the main character, a kid, got a coin from a mysterious shop that gave him the ability to turn everything that touched his lips to chocolate, and for some Fridge Horror, the cover of the book has the kid kissing his mother on the cheek, and you can see that she's turned into chocolate from the head down to around her shoulders.
The song King Midas In Reverse was written by someone who didn't understand the Midas myth and all its implications; the song is about somebody whose touch turns things to dust. The writer failed to realise that turning things to gold at a touch is just as bad; how would such a person eat or drink?
Varied by singer/songwriter Lloyd Cole in the lyrics of "Perfect Blue": "Whatever I touch turns blue..." note "blue" as in "depressive, melancholic..."
Eos, a Greek nymph, gifted her lover Tithonus with eternal life but not eternal youth. She meant for him to have youth, but somewhere in the fine print that got left out. He turned into a cicada in the end, slowly withering away.
Selene, titaness of the moon, fell in love with Endymion, a prince. Learning from the aforementioned example of eternal life without eternal youth, she wished for her lover to remain forever in the state she first saw him, obviously meaning for him to stay a healthy and strapping young man. What she forgot, was that she first saw him while he slept...
Sir Gawain's strength increases with the sun, making him nigh invincible about noon. Unfortunately, it also decreases with the sun, leaving him sapped of energy later in the afternoon and unable to work nights.
Actually even more horrific than one might think at first, since the British Islands are notoriously cloudy. If the ability means "amount of sun = strength), then only very rarely would he have full strength.
Prometheus was immortal, which was cool until the whole eagle-eating-out-his-liver-daily thing. Eventually, Hercules did pass by and unchain him though.
More dramatically, Prometheus's gift of fire to man was this, as well. Previously, man lived in an idyllic-but-primitive state, part of the gods' attempt to make sure that this time they wouldn't kill each other. While the fire from Olympus did inspire and elevate them, it also created violence and led all the social problems of the world.
According to some versions, Pandora being presented to mankind by the gods after the fire was stolen, and then her opening the stupid box/jar, led to the problems of the world. Can also be a case of lousy blessing in that Pandora was gifted by all the gods to be the perfect womannote Greek style -then Zeus blessed her with endless curiosity, without the brains or common sense to restrain or direct it.
Chiron was also immortal, which was fine until he was accidentally shot with a poisoned arrow. He was left in burning unending pain until he sacrificed his immortality to allow Prometheus to go free (if he hadn't not even Hercules could have freed Prometheus).
Servants in the Muslim paradise are said to be "boys blessed with eternal youth". Imagine being twelve or thirteen years old forever, eternally waiting on martyrs with dozens of virgins each, whom they are not expected to share with the help. Those boys' wanking arms are going to be really tired after a century or two!
Speaking of Islam: You may wonder, how can a men that enter paradise can satisfy all the virgins that he will get? Well:'It is quoted by Ibn Kathir, in his Qur'anic Commentary, the Tafsir ibn Kathir, and they are graphically described by Qur'anic commentator and polymath, al-Suyuti (died 1505), who, echoing a hasan hadith from Ibn Majah, wrote that the perpetual virgins will all "have appetizing vaginas", and that the "penis of the Elected never softens. The erection is eternal".' In case you missed some of that: Men in islamic paradise will have eternally erected penises.
From Irish mythology, Diarmuid O'Duibhne was blessed by an unknown fae being with a spot on his face that would cause women to fall in love with him just by looking at him. Sounds great, until the woman who's supposed to be marrying his king falls for him instead and forces him to elope with her, causing said king to chase after him with all his army. Which was mostly made up of Diarmuid's old friends, who didn't much like the situation either. It didn't end well, but it did inspire the later legends of Tristan and Isolde, and Lancelot and Guinevere.