Which gambits get good results if they're accepted, but lead to poor results for the gambiteer if they're declined? Some gambits are playable whether the gambit is accepted or not, such as the Queen's Gambit, the King's Gambit, the Evans Gambit and the Budapest Gambit. Some are just dubious (the Elephant and a host of others). Some are playable if they're accepted, but the Declined forms are just worse for the gambiteer. Which gambits fall into this category?
One example is the so-called Calabrian Countergambit: 1.e4 e5 2.Bc4 f5. After 3.exf5 Nf6, black seems to be fine. However, white should ignore the gambit and play a developing move: 3.d4 (Bologan-Bednarek), 3.d3, 3.Nf3 or 3.Nc3.



      [StartPly "4"]

      [FEN ""]
      1.e4 e5 2.Bc4 f5 3.exf5 (3.d4)(3.d3)(3.Nf3)(3.Nc3) Nf6




Another example is a line in the exchange variation of the Ruy Lopez: 1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Bxc6 dxc6 5.0-0 Bg4 6.h3 h5. Accepting the sacrifice with 7.hxg4 is clearly too dangerous as it opens the h-file for the black rook. White is probably slightly better if he declines the sacrifice with 7.d3 (Naiditsch-Mons).



      [StartPly "12"]

      [FEN ""]
      1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Bxc6 dxc6 5.O-O Bg4 6.h3 h5 7.hxg4 (7.d3) hxg4