There are times when a player has to be wary and take a situation fully into hand before advancing, and then there's times when you've made a perfectly normal hallway that your players refuse to go down because they think it's going to collapse in on them.

Story time! Tell a short story or two of a time when you either way overthought or underthought a situation.

I have planned, at level 2, a thousand man assault on a goblin camp. A thousand was all I managed to convince out of the crowds. The rest of the party thought it was hilarious. Basically, one of the goblins had kicked me in the shin and stolen my money, a whopping 20 gold.

In a campaign I ran, I wanted to have a sort of mini-dungeon in the middle of a city, without needing my players to have to go up against the guards- solution? Overly elaborate sewers with a bandit base inside! (steampunk setting, so the super-fancy sewers were explained away as 'wastewater treatment technology')

Now, in order to design them, I figured out a nice, simple repeating pattern of 6 foot wide hallways/tunnels, and tesselated it through several large ovoid rooms, which each had a 'moat' of sewage and some ledges with 4 doors encircling them.

None of these rooms had any traps (aside from the last one with the bandit camp) and you could circumvent the entire maze-y bit by just walking around the side. It was an entire two sessions before the players managed to find their way through and realize how things were set up, and the first few 'you see a hallway, 6 feet wide, and x feet in length' over and over. So paranoid, they were.

Oh god I can only imagine the backlash I'd get if I'd tried to do something like that with my players, they already accuse me of playing Hallway Simulator© in my games as it is, doing an entire 2 sessions in one would have gotten them throttling me over the internet.

My old DM, in every single campaign he ran, always had a long hallway with two pits filled with deadly stuff except for a 3-by-3 square in one on either side and tiny ledges on the walls. Everyone who knew them dreaded the things, because there was always something different about them that would get most of the party killed.

The one I remember best is where there were either snakes or a poisonous gas in both pits, with the 3-by-3 square that was empty, and a matching noticeable square on the floor we were on. A giant boulder started rolling towards us, so naturally all of us who could fit went down to the empty square, with a couple of us being crazy and jumping over the thing.

Turns out the square on the floor was a spring, which launched the boulder down into the hole we had gathered in, killing a good chunk of the party. If we had all just stayed put, it would have done that before it got to us, and we could have just walked through. It was probably the worst experience with that hallway I had.

...no Reflex save to dive aside, or did everyone tank the roll?
(I think I'll steal this one for my next Christmas dungeon, if you don't mind. Thanks!)

I think instead of a reflex save they got the choice to not stand in the wrong place :P

One time while I was running a game the players got a bit silly on me.

<Me> "At the end off the hallway you see a closed door."
<Part>*15 minute debate about how to open the door, involving suggestions of explosives, lockpicking, and bashing it with their weapons.*
<Me> *sigh*"Perhaps one of you should try the handle."

Happens all the time in my party. Now when we see a door it looks more like this.
Party: Is it closed?
GM: Yes.
Half an hour later.
Party: Is it locked?
(It's way more funny in my native language as there is only one word for locked and closed)

People always want what is best for themselves. Especially if those people are Player Characters being gamed by a jerk, or worse, a consummate In Character Role Player.

When has your entire party been screwed over by a player being true to his desires and not to the common good?

I think this one counts as doing something both the character and the player would want to do.

So my character is an aspiring super villain (think Team Rocket-esque not-really-evil inneffectual villain) with a history of working with machines. I even had a "summon doombot" attack.

Anyway, the big bad had created a number of artificial, mechanical superponies, one for each of the four classical elements, and was using them as her minions. IIRC, one had already switched sides, and we were about to have a showdown with another one. I can't remember the exact circumstances, but for whatever reason, the big bad showed up, "killed" her minion (read: deactivated), and left. I think we may have pinned the minion down and were trying to get information out of her, so that might have been why.

So the rest of the party is getting ready to leave, 'cause, you know, we're done here, right? Well, here's my mechanically inclined villain-wannabe, along with a perfectly good robot that's a perfect simulacrum of a living pony, abandoned by her former mistress. Honestly, what did you think I was going to do? Because aparently no one else, including the DM, saw what came next. I grabbed that horsebot and took it with me.

Needless to say, this lead to some shenanigens later. This particular robo pony was not a Nice Person (TM). In fact, she was Grade A A-hole material. The first thing she did when I reactivated her was try and screw with the party. The NPCs in our party hated her so much that they destroyed her original body (much to my character's distress). The entire party even got together and destroyed her brain chip, although one of her sisters had apparently felt merciful and swapped it out with a blank chip first.

I eventually made it one of my goals to build her a new body and reform her, although the campaign ended before I could complete her body.

I guess the moral of the story is: don't expect adopting a villain's discarded minion to result in a Heel-Face Turn, and don't expect the party to forgive said minion because they [alledgedly] work for you now. Despite that, I don't regret what I did, but a little more wisdom and caution would have been helpful.

Basically we were playing a module where we were probably supposed to help out a revel group, but instead we wanted to take bounties on them. We found the leader (who didn't actually have any class levels) and the other guys on the party wanted to find out where the other leaders were. But my guy just thought we were getting the leader, so he sliced her in half. They weren't too happy with that.

As a break from a very long campaign I wound up running a one-shot with a simple homebrew system. Our heroes had been sent to help out at a research facility, only to find everything had gone Resident Evil by way of Metal Gear Solid. Battling their way through the underground facility they made their way through to a watchtower posted at the fringe of the town the facility was in.

After a dramatic boss fight I was ready for the next thing I had planned. See, all the bosses they would face were inspired by Metal Gear Solid ones, and the 'Sniper Wolf' equivilant was posted in the long storage room they had passed through to get here. The idea of the fight was rather than it being a 'battle', was for them to try and carefly proceed through the building, dodging sniper fire, to reach the boss and take her out.

The party, however, deciding that their charecters REALLY didn't want to go back into the dark, blood stained, monster infested, undgeround facility, decided to simply scale the cliff by the watch-tower and walk back across the surface, inadvertantly setting them up to completely by-pass the boss fight all-together.

To be fair, it's not an unreasonable to assume that Krieg would be more or less unable to fight back for the two turns he's preparing to fire. If it takes twelve seconds to fire, Luffy has plenty of time to reach, him and at least stagger the guy, possibly interrupting his attack and forcing him to start over. None of these ideas are unreasonable guesses.

Or break out the grapple rules, or sunder the darn thing, or sunder his facemask... either way, no Mh5 for him

Theoretically, he could fire it and dive into the water. Get out of range underwater before surfacing. Luffy sure can't do that.

with that Armour I'm pretty sure krieg couldn't either

If Mh5 has to be inhaled, Luffy could dive under the water…WHILE HOLDING ONTO A SHIP.

Then, he uses the ship to launch himself clear of the MH5.

Just saying.

Aren't there a ton of chefs and customers around as collateral damage?

Like Luke cares about NPCs.

Nah. If luffy dive underwater, he will lose his strengh, including his grip on the ship.
Plus, Luke did try to pay at least some attention to NPCs (or at least stay in character). Cory is the one who don't care at all.