You could decide that the item has an inherent Mercurial effect.
The idea with MM is that you know Magic Missile, I know Magic Missile, and Bob knows Magic Missile -- but we each have a different version under our belts. If Bob teaches his version to Mary, she'll cast it with his MM effect OR with a new effect because she can't quite do it the way Bob does (six-fingered devil that he is...).
I think having a Mercurial effect for an item brings a lot of distinction to an item. It's not run-of-the-mill...
Gnome Boy (a.k.a. "Jon") • DCC play-tester @ DDC 35, Feb 2011. • Beta DL 2111, 7:00 AM PT, 8 June 11.Playing RPGs since 1977
• Quasi-occasional member of the Legion of 8th-Level Fighters.
Link: Here Be DCC Monsters