user8648: Are there any gambit lines in which declining the gambit is a blunder or at least can be considered a misplay?

user10380: Some gambits whose best response is to accept.
1 e4 c5 2 d4 cxd4 4 c3: Sicilian Defence, Smith-Morra Gambit
1 d4 Nf6 2 c4 e5 3 dxe5 Ng4 4 Bf4 Nc6 5 Nf3 Bb4+ 6 Nbd2 Qe7: Budapest Gambit
1 e4 e5 2 Nf3 Nc6 3 Bb5 g6? 4 d4 exd4 5 Bg5: Ruy Lopez, Fianchetto Variation
1 e4 e5 2 Nf3 Nc6 3 Bc4 Nf6 4 d4 exd4 5 0-0: Italian Opening, Two Knights Defence
Does it count as a gambit if the gambited unit is captured but not immediately after it is moved? (You could argue that the gambiting player gambits a unit by not defending it.) If this counts:
1 e4 e5 2 Nf3 Nf6 3 Nxe5 d6 4 Nf3 Nxe4 5 d4 : Petrov Defence
1 e4 e5 2 Nc3 Nf6 3 f4 d5 4 fxe5 Nxe4 5 Nf3: Vienna Gambit Declined
1 d4 Nf6 2 c4 e6 3 g3 d5 4 Bg2 dxc4 5 Nf3 Be7 6 0-0 0-0: Catalan Opening
I don't count captures where the capturer is immediately recaptured, because these are termed exchanges rather than gambits. But the question "should I start this exchange?" is just as valid as "should I accept this gambit?".
You might count the Danish Gambit
1 e4 e5 2 d4 exd4 3 c3
on the basis that White gambited wPd4. However, Black might well have expected an exchange on d4; the mainline is the Centre Gambit 3 Qxd4.

user15223: Sure, for example the Halloween Gambit. This is not a sound gambit at top levels of play, but if White plays it, Black must accept. The alternatives are even worse.
[FEN ""]
1. e4 e5 2. Nf3 Nc6 3. Nc3 Nf6 4. Nxe5 Nxe5 (4... Nxe4 5. Qh5 g6?? 6. Nxc6 wins) 5. d4 (5...Nc6 6. d5 Ne5 7. f4 Ng6 8. e5 Ng8) (5...Ng6 6. e5 Ng8 7. Bc4)

Black's best move is 4...Nxe5. Every other move except 4...Nxe4 gives away a pawn for nothing, and 4...Nxe4 allows White a dangerous initiative after 5. Qh5.

8648: <p>Are there any gambit lines in which declining the gambit is a blunder or at least can be considered a misplay?</p>
 10380: <p>Some gambits whose best response is to accept.</p>
<p>1 e4 c5 <strong>2 d4 cxd4</strong> 4 c3: Sicilian Defence, Smith-Morra Gambit</p>
<p>1 d4 Nf6 2 c4 <strong>e5 3 dxe5</strong> Ng4 4 Bf4 Nc6 5 Nf3 Bb4+ 6 Nbd2 Qe7: Budapest Gambit</p>
<p>1 e4 e5 2 Nf3 Nc6 3 Bb5 g6? <strong>4 d4 exd4</strong> 5 Bg5: Ruy Lopez, Fianchetto Variation</p>
<p>1 e4 e5 2 Nf3 Nc6 3 Bc4 Nf6 <strong>4 d4 exd4</strong> 5 0-0: Italian Opening, Two Knights Defence</p>
<p>Does it count as a gambit if the gambited unit is captured but not immediately after it is moved? (You could argue that the gambiting player gambits a unit by not defending it.) If this counts:</p>
<p>1 e4 <strong>e5</strong> 2 Nf3 Nf6 3 <strong>Nxe5</strong> d6 4 Nf3 Nxe4 5 d4 : Petrov Defence</p>
<p><strong>1 e4 e5</strong> 2 Nc3 Nf6 3 f4 d5 <strong>4 fxe5 Nxe4</strong> 5 Nf3: Vienna Gambit Declined</p>
<p>1 d4 Nf6 <strong>2 c4</strong> e6 3 g3 d5 4 Bg2 <strong>dxc4</strong> 5 Nf3 Be7 6 0-0 0-0: Catalan Opening</p>
<p>I don't count captures where the capturer is immediately recaptured, because these are termed exchanges rather than gambits. But the question &quot;should I start this exchange?&quot; is just as valid as &quot;should I accept this gambit?&quot;.</p>
<p>You might count the Danish Gambit</p>
<p>1 e4 e5 2 d4 exd4 3 c3</p>
<p>on the basis that White <em>gambited</em> wPd4. However, Black might well have expected an <em>exchange</em> on d4; the mainline is the Centre Gambit 3 Qxd4.</p>
 15223: <p>Sure, for example the Halloween Gambit. This is not a sound gambit at top levels of play, but if White plays it, Black must accept. The alternatives are even worse.</p>
<pre><code>[FEN &quot;&quot;]
1. e4 e5 2. Nf3 Nc6 3. Nc3 Nf6 4. Nxe5 Nxe5 (4... Nxe4 5. Qh5 g6?? 6. Nxc6 wins) 5. d4 (5...Nc6 6. d5 Ne5 7. f4 Ng6 8. e5 Ng8) (5...Ng6 6. e5 Ng8 7. Bc4)
</code></pre>
<p>Black's best move is 4...Nxe5. Every other move except 4...Nxe4 gives away a pawn for nothing, and 4...Nxe4 allows White a dangerous initiative after 5. Qh5.</p>
