Twist: You think this is the legendary lost crown of Foo? Some rotten trash you grabbed in a dungeon just happens to be the thing you've been looking for all this time? Pull the other one! It's been so ravaged by time that none of the markings or engravings are clearly visible. Best you can hope for is that some merchant will buy it off you for scrap.

Even if the PCs think this is the lost crown of Foo, only the kingdom's last grandmaster artificer can conduct a conclusive test. Assuming you even find them, it's not like they take appointments from any dirty old adventurers off the street.

Going by your entire comment alone.

I have juuust the right youtube video for you: https://www.youtube.com/watch?v=EkXMxiAGUWg

I'm extremely naive when it comes to tabletop RPGs

Is there any kind of "plot says no" response to magic? Something like the doors in oblivion where you need a key to unlock

Really, what the DM says goes. So if you want to be boring you can just say it doesn't work for some reason. The answer above re: pivoting to it being a powerful illusion spell or something so there is a reason the spell didn't work is a lot more compelling and interesting imo

That makes sense! I've always wanted to run a campaign (even though I've never really played) so I try to take guidance from stories like these

Thank you!

there's two answers to this question, one is mechanical and one is social. you as the DM can tell the players no not now, and they can't do anything about it, but that doesn't mean they won't try to do something about it, which depending on the group could be an issue.

so in this scenario a good DM could whip up some misdirection, for example set up a traveling artificer who just passed through town a couple weeks back and who the players could track down as a lead - conveniently in the direction of the main quest objective.

this is hard to do on the spot.

We don't do that here. The GM provides the model of physics the players accept and expect. If the GM just says "nah" when stuff is inconvenient, players don't know what to expect, and the world becomes inconsistent.

A big part of the GM's fun in TTRPGs is improving off that. Players always ruin my plans, but that's part of the game.

You could also just have it work and go with whatever follows from it though.

I believe you should have a plot prepared but you also shouldn't be afraid to adapt it if the players do something unexpected.
It's more work, but in my experience players can usually smell when you're just trying to block them. And they will derive fun from having found out your plans early (which is totally ok to tell them).

If you've railroaded your campaign that much you're a bad GM. It's not your story, it's your players story.

"You can certainly try"

Ime, players are entirely willing to accept an extremely short session just so I can prep and set back up after they throw me a massive curveball. If you're capable of doing it on the fly, that's great, but I'm not and my players usually understand.

Had a twelve minute session once because I forgot I gave the party a foldable boat like three months ago on a whim, and they used it to skip the next ~3 sessions of content. I had an entire thing setup where they'd help a dwarfhold hunt a dragon, and had started on some city-based intrigue in the next area.

I just leveled with them that I had not even slightly expected this session to go this way and had nothing prepped so we'd stop early and pick it up next time.

The crown completely disintegrates, as it was rust all the way through

Sorry, Mario, the real crown is in another dungeon.

I once fast-forwarded a complex plot through a GM-sanctioned bit of fluff.

The party had been invited by their uncle who turned out to be recently murdered when they arrived. Of course they investigated. At one point I had my character wrote a letter to the rest of the family to inform them of what was going on. I actually produced the letter as a handout. Since I had no idea about the date I asked the GM and he told me to pick anything in summer.

The GM s happy with the handout and it was deemed canonical.

A few sessions later he noticed that I had picked something ahead the end of the summer and the bad guys' plot was about to kick off at a specific date right after summer ends. So suddenly the adventure went from "careful slow-burn investigation" to "mad rush to the location of the finale".

Oops.

Couldn't they have gone the other route and made the villain's plans a year later? But sounds like it was a lot of fun the way it was run!

The idea was to have some kind of urgency but only once the players were far enough to understand the basics of what was going on. To that end, the date was supposed to be vague so that the GM was free to say "you figured out that the ritual will happen right after summer ends – which is in less than a week".

Then he forgot that the timeframe was vague when I wrote the letter and told me to pick a date.

Unfortunately, this cut out a side plot where our party would've hired another party to hunt down some artifact. That artifact retroactively got downgraded to a red herring for time reasons.

On the other hand, we got an absolutely precious scene where the one party member who wasn't magic-affine and didn't want to be involved with any supernatural stuff had to ride an unnaturally fast six-legged half-demon horse in order to catch up with the bad guys.

Also, it cut down on all the "three wizards and a vintner have breakfast and discuss the state of the investigation" episodes. We had a lot of those.

Rollercoaster are fun yet have rails.

Are you even a GM to allow yourself such snap judgment? But for you know, we GM/DMs are not your employees RPGs are a group collaboration.

How is this in any way railroading?

the real crown was the XP we collected along the way

Retconing things to protect muh precious twists is not compelling, though, it's just base metagaming. The unwavering plot is the GM equivalent of the 8 page main character syndrome PC backstory. If I found out my GM was doing that, they wouldn't be my GM anymore.