Suggestions needed: highly situational/nearly useless magical items
I'm running a campaign, and one of my players has access to a storage library of magical items that are perceived to be mostly useless because they provide a passive that most people wouldn't benefit from, or they provide a function that most people can't or would never use.
For example, one of the items in storage is an Aeon Stone (Gold Nodule), where the stored language is Common. Being able to speak common is critical, but just about every humanoid creature can already speak it, so most people would perceive this as useless. In my game, this passive benefit has proved to be extremely valuable.
Another example I can think of is an item makes a person or object invisible but only if no one is looking at it. I don't think such an item exists in any official sourcebook, but this is the kind of function I'm looking for.
So, I'm looking for items like this. Any suggestions?