A character beyond what UNICODE restricts itself: &#x20FFFF; - shows like
Something below the max limit, valid, but unassigned 𰀁 - shows like
Something that's an invalid UNICODE character -  - shows like
The REPLACEMENT CHARACTER i.e. � - shows like �
Now, what the above will do inside the storage, I don't know, I'm just trying to go forth and BREAK those minions!
(hmmm, the "preview" looks like they are totally kicked out, not replaced by the replacement character [wikipedia.org] - won't cry over the loss of it)