Still, there are some pretty useless magic items - or at least, items that have a very singular and obscure purpose (these are often found in adventures instead of core rulebooks.)
Going the other direction, there are items, such as the Wand of Wonder, that no one in their right mind would ever use in the midst of combat unless all was already lost, as the potential to screw is greater than that of a positive result (I really should do another crowd sourced Wand of Wonder effects post - it has been a number of years since I did that previously.)
Here's an example I wrote a few years back. Maybe not useless, but not all that useful either:
Beer Bear - This appears to be a child's stuffed bear of smallish size. If one tilts the bear's head back and makes a poring motion with the bear, cold beer (up to the amount of a pint) flows forth. The type of beer that the bear dispenses depends on the color of the bear. Black bears dispense stout and brown bears dispense a pilsner. The bear will dispense beer up to 8 times in a single day.
Hard to beat the image of one of the PCs going to sleep in a dungeon, holding his Beer Bear tight to him, out of fear another party member may take off with it ;)So, what is the most useless magic item you have come across in an RPG campaign?