That's love players, your DM is trying to save you :)
That's why it's important to assess the tone the DM is using when they say that.
Do they sound defeated? You're probably about to derail away from something planned to something that will have to be fully improvised on the spot, reconsider for sanity's sake
Do they sound excited? You're probably about to trigger a really cool trap, full steam ahead
A good DM will also say that to mess you up when you're about to solve something.
I know a DM that created a fake puzzle room, and asked that for everything. They were in the room for an hour before realizing that the door was unlocked
I presented my party with a locked door to which they has the key. Took 20 minutes and a chime of opening to get through.
This was not a puzzle. I'm afraid of giving them a puzzle.
Indeed. I often do that kind of trick, just to keep them on their toes :)
Art by @ahdok of course. I am talented but not in drawing anything remotely fun to watch :)
If you have to ask if they're sure you've either not described a situation clearly enough, or you're giving them meta knowledge of an upcoming complication - and if you need to stop them from triggering a complication you shouldn't have put that complication there in the first place.
It's a popular and well known phrase, but it needs to be acknowledged that it's a crutch to make up for DMing mistakes that are in danger of killing a PC, not a way of protecting them. You shouldn't get to the point where you need to warn a player they're about to get their PC killed - if they are taking a potentially suicidal action they should already be aware and have made their decision based on it.
It’s a popular and well known phrase, but it needs to be acknowledged that it’s a crutch to make up for DMing mistakes that are in danger of killing a PC
You say that like if players always did good and reasonable actions even with every piece of knowledge in their hands
Oh, I'm not pretending that even the best players aren't a bunch of free range maniacs heart, it's just that it's rare that a player will actively choose to do something that is likely to kill them - They might set off a chain of events that digs them into a hole and leads to their death over a series of decisions, but if they're aware of what's going on they generally shy away from single "are you sure you want to do this" moments with a real risk of causing immediate death.
Since everything that ever happens is always the DMs doing, then yeah every single death is the DMs fault per default.
Now, lets pretend that were talking about a decent DM that isnt trying to actively kill their players, is giving them a good amount of info, but is still playing with death possible and dangers that wont be steamrolled.
From this point, for you, when is it the DMs fault and when is it the player's fault ?
And the next question is, when would it be appropriate to say the famous saying without it being a DMs crutch to cover "their" "mistakes" ?
I'm not sure where you got the idea I think that every death is the DM's fault? I said this is a crutch for the specific situations when it is the DM's fault a PC will be killed. If a PC stays in the front line on low HP, it's absolutely not the DMs fault if they die, that was a decision the player made. If someone just has a really unlucky turn and gets critted into oblivion it might not be anybody's fault, just the luck (or lack thereof) of the dice. It's the DMs fault when the players are missing information that would let them make informed choices.
As for your second question, you could have it said by a character in game, but coming from a DM to a player it's a loaded question designed to stop them from taking actions. There are only 3 scenarios where it's appropriate to say seriously: firstly, when the player has too little information and is going to take an action without properly understanding the context. In this scenario it's more appropriate to ask what they think is going to happen or what they're trying to achieve to iron out the misunderstanding, rather than let them bumble about trying to work out what their mistake is.
Secondly, when a player is about to walk into some kind of hazard they had no way to know about or reason to think about - this is at worst gloating that they couldn't read your mind, and at best a sign you shouldn't have put the hazard there.
Thirdly, there are times when a player is fully aware of the situation and is taking a suicidal action - you don't need to ask if they're sure, they already decided. They might be intentionally trying to get the character killed, trying to have an epic character moment, or trying to set off a trap because the last 30 minutes have been pretty boring and they feel like throwing some excitement into the mix, but your warning won't change their decision, because it was made with that danger already in mind. There might be an underlying problem to deal with, but you're telling them something they already know.
Ultimately if you feel a need to say it you're better off thinking about or asking them about why they're doing it, and if it's not character shit solving the issue that caused it.
Humor, jokes, memes about TTRPGs