For example, when you get those weird chewy caramel things on Halloween; given out of kindness but really no good. I'm thinking of something that is a "false gift", almost like the inverse of a blessing in disguise (which this thread discusses, but none of those are what I'm thinking of).
The intentionality of the giver is not so important as the properties of the thing itself: it is supposed to be good, but really isn't.
A quick answer that comes to mind is "White Elephant".
See Wikipedia White Elephant
A white elephant is a possession which its owner cannot dispose of and whose cost, particularly that of maintenance, is out of proportion to its usefulness. In modern usage, it is an object, building project, scheme, business venture, facility, etc., considered expensive but without use or value.1
There is also a party game in which people set out to intentionally give each other white elephant gifts.
One might also consider
Poisoned Chalice
as suggested here, or from that same page:
A curse in disguise
It's not standard or a set phrase, but it should be easily understood.
As an old trick-or-treater who got more than her fair share of those yucky chewy caramel thingies, I can suggest "dud" to describe them and other like-detested treats.
The first definition for "dud" fits this usage - per the Google dictionary:
noun: dud; plural noun: duds
- a thing that fails to work properly or is otherwise unsatisfactory or worthless. Ex.: "a high-grade collection, not a dud in the lot"
synonyms: failure, flop, letdown, disappointment, loss-maker; More informal washout, lemon, no-hoper, nonstarter, dead loss, clunker "their new product is a dud"
For a little historical background (from the 1960's Midwest US), Halloween candy could be broken down by "quality":
Best - "full-size" candy bars!
Next Best - "fun-size" candy bars
OK - Other candy - includes Dum-Dum suckers, Smarties, Milk Duds (do not confuse with plain, old duds!) and finally...
Duds - Aforementioned chewy caramel thingies, apples, pennies, weird, wrapped lozenges with foreign words on them, anything else that is not candy.
Edit: In usage, "Hey! The Robinsons are giving out full-size Hershey bars! Don't bother with the Fluges' - they're giving out duds!"
A scheme or instrument for causing death or harm, especially one which eventually brings about the downfall of its creator; something which is initially regarded as advantageous but which is later recognized to be disadvantageous or harmful.
Beware of Greeks bearing gifts
The term "well meaning" might be suitably applicable, as in, "a well meaning gift." I know you said that the intentions of the giver were not so important, but despite that, the term carries the implication that the gift itself was of poor quality.
When 'good deeds' do no good, they (idiomatically) 'kill with kindness'.
Kill with kindness : Overwhelm or harm someone with mistaken or excessive benevolence.
For example, Aunt Mary constantly sends Jane chocolates and cake and other goodies, even though she's been told Jane's on a diet-nothing like killing with kindness . This expression originated as kill with kindness as fond apes do their young (presumably crushing them to death in a hug) and was a proverb by the mid-1500s.
