win/win solution
A total bu**s**t course of action agreed upon by two or more parties; supposedly it comes with benefits for all concerned, but in reality the plan generally ends in disaster, often for everyone involved.
A classic case of a win/win solution "gone sour" --- literally --- would be when a commercial farming-outfit approaches a nearby cash-strapped community to ask if they can pay them some much-needed revenue to dump organic waste in an uninhabited area of the municipality... at first blush it may sound like a good deal, but of course what usually happens in reality is that said waste "stinks to high Heaven" so much that the townspeople angrily vote to close the dump-site, forcing the mega-farm to look elsewhere for a dumpsite, and obliging the town to pay astronomical sums to have the already-dumped waste carted off to be disposed of.