I did exactly as you described recently and you are right that there seems to be no trace to go back to that function to undo it. After talking to several Mac users, including store staff, it got resolved by trial and error by an Apple Store senior support person; he expressed himself that it was an odd solution to the problem and that the key mapping may be faulty but it worked.
None of the people I talked to are aware of this capability to alter the function of the right shift key; if it wasn't for your post, they would have convinced me that there was no such function. Anyway, this is what he did:
- go to System Preferences, then Exposé/Spaces.
- in Exposé, for the "Application Windows" setting, choose "-"
