When you cast a card using flashback, it is exiled instead of going anywhere else when it would leave the stack:
702.32a Flashback appears on some instants and sorceries. It represents two static abilities: one that functions while the card is in a player’s graveyard and another that functions while the card is on the stack. “Flashback [cost]” means “You may cast this card from your graveyard by paying [cost] rather than paying its mana cost” and “If the flashback cost was paid, exile this card instead of putting it anywhere else any time it would leave the stack.” Casting a spell using its flashback ability follows the rules for paying alternative costs in rules 601.2b and 601.2e–g.
Unexpected Results will be exiled instead of returned to your hand, even if you reveal a land.
Edit - I'll elaborate: normally, a spell is put into the graveyard after it is finished resolving. A flashbacked spell would be exiled instead at that point. However, in the case of Unexpected Results, its own effect attempts to return it to your hand as part of its resolution. As such, an Unexpected Results that was cast using flashback will be exiled instead of being returned to its owner's hand at that point of following its effect.
I think because of the breadth of flashback's rules text that you'd have to exile it. Flashback's rules essentially say that any time the card leaves the stack it gets exiled instead of going elsewhere. Since moving to your hand would be leaving the stack, it gets exiled instead.
The only argument I can see would be that Unexpected Results includes a replacement effect that you might might be able to choose over the flashback replacement effect, but the moving to your hand language looks like it's not a replacement effect, so no dice.
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