You'll never test enough for the end user

as the old programmer joke goes:

a programmer walks into a bar, he asks for:

-> 2 beers please

-> 9999 beers please

-> -5 beers please

-> π beers please

satisfied with the results he leaves and invites the first normal user. The normal user goes up to the barman and asks:

"sorry where is the bathroom?"

the bar explodes

This has been animated on Youtube several times, so instead of linking to a video, here's a search for all of them.

A long time ago a fellow GM taught me a great rule: Every problem that you introduce should have at least three possible solutions; two that you came up with yourself, and one that your players will find, and which you could never have imagined.

Players: "What about that item you gave us in the third session but never used? This must be what it was for all along, right?"

DM to self: What the hell are they talking about?

So one night we’re playing a modern era game and the BBEG and her husband are “holding court” at a very expensive night club. So all the PCs get dressed fancy, rent a limo, and bribe the bouncer heftily to let us in. A quick persuasion check and we’re in and snooping around. The GM wads up about a whole notebook. “I had plans for you to come in through the skylight, sneak through the kitchen, break in through the fire exit, even find a secret door to the basement. I never thought you’d just go in through the front door.”

So sometimes it’s the GM that fails to plan the simple solution.

I play Seven Days To Die (a zombie apocalypse game) and there are quests like clear out all the zombies in this place.

Some places show you the main loot room through an armoured window will before you can get to it legitimately. There will be an armoured door with half a million hit points to tell you not to bother.

So in one such game I dug through the 1000 hit point wall next to the armoured door, looted the loot, and did the zombie extermination path backwards
- This is usually Glock 9's approach if just trying to speedily loot.

Most important rule of GM-ing, do not over-prepare details, keep a vague outline and let the player figure out the details.

At best, the PC will just find another solution that the one you ignore (why fight the guard when you can bribe/blackmail them) or even they'll ignore huge part of your scenario.

Or even worse/better (depending on perspective or frustration): That guard-whose-name-is-ummmmm-Jack, which of course is short for ummmmm, Jakarth, yes, he should be here somewhere. ([Rolling dice …] Oh, yes, of course. Oh there he is. Sribble, scribble, scribble.]) That’s the one they now insist on befriending … You know, the one who is was of no importance, and who now for some reason (players work in mysterious ways) ends up being the NPC around which the next year-and-a-half of campaign revolves.

When your group is sure the DM is walking you into a trap but they were just curating a pleasant experience.

Literally every adventure where we have to enter a potentially dangerous castle/mansion/tower. We're going to climb or fly up to a higher level window because obviously if there's a trap or ambush, it'll be at the front door.

I keep wedging things into latches so doors can’t lock behind us.
I played an aarakocra in a campaign with a brand new DM once. I triple checked and told them some of the difficulties of having a flying character and I still got greenlit. Very first quest I flew to the top of a mansion and threw down some rope. Poor DM never saw it coming and I derailed the entire thing lol. I got up to all kinds of stuff that campaign and the DM did eventually figure out how to work with my nonsense

Look, I just think of the problems. They have to think of the solutions.

As a teenager I worked on a restaurant.

One day I went in without any sleep and with a lot of weed.

Boss sent me down to get sugar for the pancake batter.

Spent like 5 minutes trying to get the package open. Ended up just cutting the top off with a knife.

Turned out I opened a salt cylinder.

Classic, it never fails that the party will latch onto some innocuous detail and lose the plot. They're always getting derailed so ea-

If it looks like a carton, expect your party to open it as a carton - don't expect them to go searching for future microplastics.

You are aware that the inside is lined with plastic anyway?
- Doesn't seem like a good reason to use 10x more plastic,.
- ...typically they're coated with food-grade wax...
- Pours half a cup of salt on my food
  
  Why did you add 10x as much salt and ruin my food?
  
  You are aware the inside of your food had salt anyway?

At least the carton was not destroyed in the process.

I half expected to scroll down and see a straw poking in the side labeled "the party's rolls" or something to that effect