I have a theoretical answer that hinges on a technical detail that I cannot confirm is true, but it's something.
Here's the deal. If A and B meet, and B then changes their StreetPass Mii to C, then the next time they meet, A will meet C for the first time while B/C will meet A for the second time. This should trigger the "am I fantastic?" on B/C's system while leaving A in the dark.
So, this is what I think could happen:
- A meets B. B meets A.
- A meets B again. B meets A again. Both elect to send a fantastic the next time the two meet.
- B changes to C.
- C meets A. C sends "I think A is fantastic".
- A meets C. A receives a fantastic despite never having seen C before. A's "I think B is fantastic" is never sent (as A doesn't meet B)...until C changes back to B, probably.
Note that this working depends on the following detail being true: once a fantastic is fired, it no longer matters which Mii sent it, only which Mii it's targetted at.