user21110: In this position I'm White and I have just played 16. d5. Stockfish labels the move as a blunder. I do understand I had better moves than this, and it's not the move I would play if I were to get this position again, but why is it a blunder? It doesn't seem that bad. My reasoning was that, having the bishop pair and better development (two minor pieces playing versus one), and Black having a somewhat vulnerable king, I could benefit from opening some lines with the pawn-break.
rn4k1/pbp1qr2/1p1pp2p/3P1pp1/2P5/2PBP1B1/P1Q2PPP/R4RK1 b - - 0 16


user4362: 16...f4 seems to win your Bg3.
16...ed 17.Bf5 dc wins a pawn.
To answer the comments:
I am not claiming that these are the computer-approved lines of best play. Just pointing out some immediate reasons why you might not want to play d5. 16...ed 17.cd f4 is likely a better version of the f4-push, because there will be no pawn on e6. 

user8300: While this is not a tactical blunder as far as I can see (maybe positional mistake is a better word), playing d5 gives away the c5 square for black's knight to occupy; it's a very strong outpost for black's knight. In the long run, the c3-c4 pawn duo is going to be weak, and especially the c4 pawn is awkward to defend. This is a common theme in, say, the Nimzo-indian, where black manages to fix and attack a weak pawn on c4; if you want to learn more about this strategic theme, you should watch some videos on the Nimzo.
Therefore, we can see that the only reason d5 is not a catastrophic positional blunder is because black cannot justify the immediate ...e5 push due to the weak f5-pawn. In fact, if black can somehow defend that weakness in the near future, white is going to be in grave danger from a positional perspective. 
So, what are the virtues of playing d5 in the given position? The only one I can think of is that it destabilizes the f5 pawn. I'm seriously questioning whether white is really well-adviced to take on e6, letting black's queen take aim at the c4 pawn.
What I immediately saw when looking at the position immediately before 16.d5 is that the bishop on g3 is rather unproductive and close to being trapped. Therefore I would consider pushing my f-pawn to give it some space. Also, I'd note that the pawn on f5 was weak, and if attacked black would be forced to respond. Therefore I would consider trying to play for an e4 break rather than a d5 break to open up the position for my bishops. I'd be seriously tempted to push my f-pawn, either to f3 (to support an eventual e4 push) or the immediate f4 push, just being more direct and aggressive, harrassing the g5 pawn for good measure (this is the computer's first choice by far, with f3 being the second. Note that white doesn't need to support e4 with a pawn to push e4, but rather only needs to make sure that the g3 bishop isn't trapped after an e4 push).
Quick addendum: I'd also like to point out that white is seriously ahead in development, which calls for more direct measures than the d5 push. Strike while the iron is hot!

user15543: The move 16. d5 is not necessarily a blunder. Although it is not good for White, it is not losing. Lichess and other chess engines sometimes see positions differently from how humans would percieve them.
That being said, analysis on chess.com deems it a a mistake. which is much more reasonable. The reason is that, in the long-term, the d5 pawn mught be lost, giving Black a +1 advantage, in material worth, at the least in some lines.  Even if White retains a pawn, the doubled pawns on the queenside will be a disadvantage that Black can use, or there will be some other disadvantage Black can create for White. 
All in all, White will have some sort of positional weakness no matter the outcome.
Here is a computer generated line that shows how Black gets the upper hand in the long-term with an extra pawn. I do not claim that this is the 100% best play, just that it shows one of the many general outcomes where Black has an advantage.
[FEN "rn4k1/pbp1qr2/1p1pp2p/5pp1/2PP4/2PBP1B1/P1Q2PPP/R4RK1 w - - 0 1"]

1. d5 Nd7 2. Rae1 Raf8 3. f4 Nc5 4. e4 fxe4 5. Bxe4 gxf4 6. dxe6 Bxe4 7. Rxe4  Nxe4 8. exf7+ Rxf7 9. Rxf4 Rxf4 10. Bxf4

Note that although f4 is technically an inaccuracy, it is perhaps best by human play.
Here is another line that shows how Black can get an advantage in other ways as a result of White’s mistake.
[FEN "rn4k1/pbp1qr2/1p1pp2p/5pp1/2PP4/2PBP1B1/P1Q2PPP/R4RK1 w - - 0 1"]

1. d5 Nd7 2. Rae1 Raf8 3. f3 Ba6 4. Be2 e5 5. e4 f4 6. Bf2


21110: <p>In this position I'm White and I have just played 16. d5. Stockfish labels the move as a blunder. I do understand I had better moves than this, and it's not the move I would play if I were to get this position again, but why is it a blunder? It doesn't seem that bad. My reasoning was that, having the bishop pair and better development (two minor pieces playing versus one), and Black having a somewhat vulnerable king, I could benefit from opening some lines with the pawn-break.</p>
<pre><code>rn4k1/pbp1qr2/1p1pp2p/3P1pp1/2P5/2PBP1B1/P1Q2PPP/R4RK1 b - - 0 16
</code></pre>
 4362: <p>16...f4 seems to win your Bg3.</p>

<p>16...ed 17.Bf5 dc wins a pawn.</p>

<p>To answer the comments:
I am not claiming that these are the computer-approved lines of best play. Just pointing out some immediate reasons why you might not want to play d5. 16...ed 17.cd f4 is likely a better version of the f4-push, because there will be no pawn on e6. </p>
 8300: <p>While this is not a tactical blunder as far as I can see (maybe positional mistake is a better word), playing d5 gives away the c5 square for black's knight to occupy; it's a very strong outpost for black's knight. In the long run, the c3-c4 pawn duo is going to be weak, and especially the c4 pawn is awkward to defend. This is a common theme in, say, the Nimzo-indian, where black manages to fix and attack a weak pawn on c4; if you want to learn more about this strategic theme, you should watch some videos on the Nimzo.</p>

<p>Therefore, we can see that the only reason d5 is not a catastrophic positional blunder is because black cannot justify the immediate ...e5 push due to the weak f5-pawn. In fact, if black can somehow defend that weakness in the near future, white is going to be in grave danger from a positional perspective. </p>

<p>So, what are the virtues of playing d5 in the given position? The only one I can think of is that it destabilizes the f5 pawn. I'm seriously questioning whether white is really well-adviced to take on e6, letting black's queen take aim at the c4 pawn.</p>

<p>What I immediately saw when looking at the position immediately before 16.d5 is that the bishop on g3 is rather unproductive and close to being trapped. Therefore I would consider pushing my f-pawn to give it some space. Also, I'd note that the pawn on f5 was weak, and if attacked black would be forced to respond. Therefore I would consider trying to play for an e4 break rather than a d5 break to open up the position for my bishops. I'd be seriously tempted to push my f-pawn, either to f3 (to support an eventual e4 push) or the immediate f4 push, just being more direct and aggressive, harrassing the g5 pawn for good measure (this is the computer's first choice by far, with f3 being the second. Note that white doesn't need to support e4 with a pawn to push e4, but rather only needs to make sure that the g3 bishop isn't trapped after an e4 push).</p>

<p>Quick addendum: I'd also like to point out that white is seriously ahead in development, which calls for more direct measures than the d5 push. Strike while the iron is hot!</p>
 15543: <p>The move 16. d5 is not necessarily a blunder. Although it is not good for White, it is not losing. Lichess and other chess engines sometimes see positions differently from how humans would percieve them.</p>

<p>That being said, analysis on <a href="https://www.chess.com/analysis" rel="nofollow noreferrer">chess.com</a> deems it a a mistake. which is much more reasonable. The reason is that, in the long-term, the d5 pawn mught be lost, giving Black a +1 advantage, in material worth, at the least <strong>in some lines.</strong>  Even if White retains a pawn, the doubled pawns on the queenside will be a disadvantage that Black can use, or there will be some other disadvantage Black can create for White. </p>

<p>All in all, White will have some sort of positional weakness no matter the outcome.</p>

<p>Here is a computer generated line that shows how Black gets the upper hand in the long-term with an extra pawn. I do not claim that this is the 100% best play, just that it shows one of the many general outcomes where Black has an advantage.</p>

<pre><code>[FEN "rn4k1/pbp1qr2/1p1pp2p/5pp1/2PP4/2PBP1B1/P1Q2PPP/R4RK1 w - - 0 1"]

1. d5 Nd7 2. Rae1 Raf8 3. f4 Nc5 4. e4 fxe4 5. Bxe4 gxf4 6. dxe6 Bxe4 7. Rxe4  Nxe4 8. exf7+ Rxf7 9. Rxf4 Rxf4 10. Bxf4
</code></pre>

<p>Note that although f4 is technically an inaccuracy, it is perhaps best by human play.</p>

<p>Here is another line that shows how Black can get an advantage in other ways as a result of White’s mistake.</p>

<pre><code>[FEN "rn4k1/pbp1qr2/1p1pp2p/5pp1/2PP4/2PBP1B1/P1Q2PPP/R4RK1 w - - 0 1"]

1. d5 Nd7 2. Rae1 Raf8 3. f3 Ba6 4. Be2 e5 5. e4 f4 6. Bf2
</code></pre>
