Watchmakers are little aliens found in The Mote in God's Eye by Niven and Pournelle. They are, in effect, gremlins. But instead of ruining technology, they improve it. The danger is they don't account for the whole, just the part they are working on. So if one was to let them loose on a starship, they might increase the strength of the engines to the point the ship will collapse or the crew will die from excess Gs. These little guys have potential in fantasy settings, one that could easily get out of control if the GM doesn't have some way of reigning them in. Here are some rules that are easy to adapt to your game:

Watchmakers are gremlins like those found in Pathfinder. I am considering having them as nature spirits gone wrong in the presence of magic items. They have a supernatural trait that allows them to upgrade magic items. Give them a potion and time and they can upgrade it to a vorpal sword. But the watchmakers do it in a random fashion and the results are very much imperfect. For every week the little guys work on an item, they add d3+1 X 1000 gp to its value. When the item is taken away from them, find what the amount is, what items are worth that amount and roll for a random one. Then apply d4+1 curses. There will never be a delusion or opposite effect so they must be intermittent functions, requirements and/or drawbacks. The GM will have to rule on any maximum value- the time the watchmakers get bored and start working on something else.

Now a player is reading this and thinking, "I can get them to make expensive items that my wizard can fix and then sell for a profit!" Sure, if your GM allows that. I wouldn't. I would rule that their form of enchantment is significantly different from arcane magic and thus any attempts to fix or upgrade their work results in much worse curses (reroll with a +50 and come up with some more gruesome results for anything over 100).

If watchmakers are used, the result is obvious- an increase in the number of magic items within the setting but the great majority will be cursed. A watchmaker family that gets into a wizard's vault can make a lot of changes before they get caught. People will trust spellcasters much more than items, though greed for magic will mean a desire that will always result in more magic for the gremlins to affect.

And, obviously, this can be applied to Mutant Future and science fiction games. The items will not be randomly altered, rather their effects will be greatly improved and the side effects will be random. Watchmakers may be aliens, alien tech or even a side effect of psionic activity (poltergeists as gremlins).