user224: Recently I was playing an online game when I moved my king with the intent of castling. However, I apparently didn't get it far enough over, and when I "let it go", the king was placed in a square adjacent to the one it had left and the rook stayed in the corner.
I made a comment in the players' chat along the lines of "That was meant to be a castle." The other player even replied with something like "Yeah, I figured." However he proceeded to take advantage of the situation, eventually winning what had been an even game up to that point.
Unfortunately, this is not the first time this has happened to me. In fact it seems to happen disturbingly often, and I've only played a few dozen online games over the past few months. Also, I've seen some puzzling moves from opponents that I now think might have been due to trouble with the equipment, rather than just poor play.
In the game above, I considered asking for a draw right after the error, especially since my opponent acknowledged that it was a technical problem, not poor play. However I wasn't sure of the etiquette for this, and also had time considerations to worry about. Would it have been considered bad form to ask for a draw? To decline one if it is asked for from an opponent in such a circumstance?

user307: On lichess.org you can ask for takeback even in rated games.
Your case is very reasonable for such request.
In my experience the option is usually available, but here is an explanation of when it is and not available.

If you go to "Preferences" - "Game behavior" you are given a choice to
  allow takebacks "never", "always" or "in casual games only".
Even if you set your preferences to allow takeback always, you will
  not have the option in games where your opponent does not allow them.
Oh, and I believe there is no option of takeback in tournament games,
  but I'm not a 100% sure about that.

https://lichess.org/qa/2557/why-in-some-games-does-the-takeback-button-work-and-others-not

user16035: The most standard response you'll see is what Michael said. However, for what it's worth, I've had that happen to me as well and if you ask, occasionally the other player will scoot a piece around until you fix your move. 
It doesn't take too much effort to plan it out so their piece is in its original spot once you've finished your "castle" (for example, they could move a bishop 2 squares over, you move your king off the back row, they move their bishop one square back, you move your rook over, they move their bishop back to its original spot, you move your king to it's standard castle position, then play continues). 
It messes up the time a little and potentially impacts a draw, but that's seldom been relevant in my cases. Having said that, you're also asking your opponent to deliberately make a sub-par move and trust you won't exploit it, not to mention pass on a winning opportunity. Lots of players won't do that for you, but it shouldn't hurt to ask. 
I've also had a few opponents slip up before and offered to let them fix it a few times now. In the dozen or so times I've offered that to my opponent, they've never taken advantage of me for it; they've always just made the move they promised. Likewise, in the dozen or so times someone's offered that to me, they've always operated in good faith as well. 
Again, this isn't the most common response, but there are some players out there who'd much rather play a good game than an easy one. For what it's worth all these matches were Elo ~1600; I don't know how that affects the competitiveness of the matches. 

user17123: That's called a mouse-slip, and it's part of playing online. Watch some of Carlsen's online play (e.g. https://www.youtube.com/watch?v=SJlOlufG-JM) and you'll see even he mouse-slips on occasion. Common etiquette seems to be to continue playing out the game and try to be more precise next time.

224: <p>Recently I was playing an online game when I moved my king with the intent of castling. However, I apparently didn't get it far enough over, and when I "let it go", the king was placed in a square adjacent to the one it had left and the rook stayed in the corner.</p>

<p>I made a comment in the players' chat along the lines of "That was meant to be a castle." The other player even replied with something like "Yeah, I figured." However he proceeded to take advantage of the situation, eventually winning what had been an even game up to that point.</p>

<p>Unfortunately, this is not the first time this has happened to me. In fact it seems to happen disturbingly often, and I've only played a few dozen online games over the past few months. Also, I've seen some puzzling moves from opponents that I now think might have been due to trouble with the equipment, rather than just poor play.</p>

<p>In the game above, I considered asking for a draw right after the error, especially since my opponent acknowledged that it was a technical problem, not poor play. However I wasn't sure of the etiquette for this, and also had time considerations to worry about. Would it have been considered bad form to ask for a draw? To decline one if it is asked for from an opponent in such a circumstance?</p>
 307: <p>On lichess.org you can ask for takeback even in rated games.</p>

<p>Your case is very reasonable for such request.</p>

<p>In my experience the option is usually available, but here is an explanation of when it is and not available.</p>

<blockquote>
  <p>If you go to "Preferences" - "Game behavior" you are given a choice to
  allow takebacks "never", "always" or "in casual games only".</p>
  
  <p>Even if you set your preferences to allow takeback always, you will
  not have the option in games where your opponent does not allow them.</p>
  
  <p>Oh, and I believe there is no option of takeback in tournament games,
  but I'm not a 100% sure about that.</p>
</blockquote>

<p><a href="https://lichess.org/qa/2557/why-in-some-games-does-the-takeback-button-work-and-others-not" rel="nofollow noreferrer">https://lichess.org/qa/2557/why-in-some-games-does-the-takeback-button-work-and-others-not</a></p>
 16035: <p>The most standard response you'll see is <a href="https://chess.stackexchange.com/a/22238/16035">what Michael said</a>. However, for what it's worth, I've had that happen to me as well and if you ask, occasionally the other player will scoot a piece around until you fix your move. </p>

<p>It doesn't take too much effort to plan it out so their piece is in its original spot once you've finished your "castle" (for example, they could move a bishop 2 squares over, you move your king off the back row, they move their bishop one square back, you move your rook over, they move their bishop back to its original spot, you move your king to it's standard castle position, then play continues). </p>

<p>It messes up the time a little and potentially impacts a draw, but that's seldom been relevant in my cases. Having said that, you're also asking your opponent to deliberately make a sub-par move and trust you won't exploit it, not to mention pass on a winning opportunity. Lots of players won't do that for you, but it shouldn't hurt to ask. </p>

<p>I've also had a few opponents slip up before and offered to let them fix it a few times now. In the dozen or so times I've offered that to my opponent, they've never taken advantage of me for it; they've <em>always</em> just made the move they promised. Likewise, in the dozen or so times someone's offered that to me, they've always operated in good faith as well. </p>

<p>Again, this isn't the most common response, but there are some players out there who'd much rather play a good game than an easy one. For what it's worth all these matches were Elo ~1600; I don't know how that affects the competitiveness of the matches. </p>
 17123: <p>That's called a mouse-slip, and it's part of playing online. Watch some of Carlsen's online play (e.g. <a href="https://www.youtube.com/watch?v=SJlOlufG-JM" rel="noreferrer">https://www.youtube.com/watch?v=SJlOlufG-JM</a>) and you'll see even he mouse-slips on occasion. Common etiquette seems to be to continue playing out the game and try to be more precise next time.</p>
