I'm usually well prepared for my sessions these days. I have enough mapped out, and enough obstacles between the PCs and the unkeyed and unmapped areas that I don't need to worry they'll outrace my prep. I haven't had to use these yet in my game, because I'm far enough ahead.
This wasn't always the case - my previous game was a wide-open sandbox with a player-driven storyline, external events they couldn't control, and fairly easy access to travel magic. They could literally spin on a whim and go somewhere I'd hinted at but hadn't had time to write up. Or finish painting the minis for (hey, I custom paint minis for set-piece brawls sometimes)? Or go to question someone who really should provide them with a (well-thought out by you) answer, but you're rather push it off until next session so you can mull it over? Or they're about to attack that NPC party and you realize you left the damn NPC party's writeup at home? Basically . . .
What happens if your PCs head right to blank spot on the map?
Here are some tactics I use to stall them, deflect them, or distract them.
Stall: A classic. Just keep them busy, and amused, but don't let them into the blank spot.
Magic Door - That door just ahead of them? Well, it's not a normal door anymore. Now it's made of some bizarre material that resists normal forcing and magical bypassing. You can't get past it without a key - a key found elsewhere in the dungeon/campaign world/over in the next town a week's travel away.
This is a useful stall because it automatically makes this section of the dungeon more interesting.
Blue Ribbon - The area is blocked with a forcefield, a force of friendly critters who just won't let you by for security reasons, or another impenetrable force . . . that yields to a special seal or title given elsewhere. Where might be obvious ("Show the seal of the duke and you can go past us") or not so obvious (A cryptic hint). Show your mettle (or your money) elsewhere and suddenly the elevator starts working and you can go W*E*R*D*N*A hunting. This is basically a Quest - and it's a Deflect approach as much as a Stall.
Dave's not here, man - Want to talk to the NPC and you're not ready? Well, neither is the NPC. Tonight is the gala ball/his mother is in from out of town/he's at a secret society meeting/he's been summoned to another plane/he's sick in bed. He's out, he can't see you, but he's left a note/message/etc. pointing the PCs somewhere else for a partial clue or a clue to something else (that you, the GM, already have handy).
Wandering Monsters are a generic stall. They're also a time-use tax, but they can also be a good stall until next session. A big nasty coming down from the corridor that ends in "I'll finish this later . . . " or "under construction" can sap resources, soak up session time, and potentially deflect players by forcing them to flee. Plus let's face it, people like fighting stuff, so it's not even much of a punishment. It's not even unfair, unless you're one of those people who thinks putting down "10 ogres are in this hallway" is fair if done between sessions and unfair if done right when the players go down the hallway.
Reply Hazy, Ask Again Later is good for information requests. "I can find out, but it'll take some time. Hit me up next time you come back from the dungeon." In other words, ask next session. Hey, I'm online every day and you didn't email me, so if you ask right before the dungeon delve and I'm not ready with an answer, well, neither is Cedric the Sage.
It's a Trap!: The heaviest of hands, here - trap them. Drop walls ahead and behind, and make sure the behind is easier to break. Keep them busy for a while and make sure the "out" is back to mapped areas.
Deflect: Turn them away from the blank spot.
Wandering Monster - again, but this time use something scary. Literally (so they must flee) or just powerful (so they should flee). A bit heavy handed, and it might tempt a TPK-inducing fight, so be careful with this.
Magic - Teleport tricks, rotating rooms, elevator rooms that dump you back up a level, confusion spells that confuse direction-finding. All of these serve to effectively deflect the PCs away from an area until you've written it up.
Detour Ahead - Block the way physically, with a barrier they can't easily move. You can have it removed later - maybe an umber hulk digs through the rock wall, the rubble gets dug out, the collapsed hallway is repaired by the dungeon cleanup team, whatever.
Distract: My favorite. Give them something else to do.
Wandering Treasure: "You hear the unmistakable clink of coins." Or a thief with a jeweled necklace sprints by. Or a prisoner tells them where a (already mapped) treasure lays. Or they find a map showing an already explored area, with some spot they missed marked on it.
Strange Sounds: It works in sneak-type video games. Make a sound, the guards look. Works on PCs, too. "You hear a door slamming shut . . . behind you." Maybe they'll go look.
Help Me, Kind Strangers: Have an NPC come upon them, and ask for help elsewhere. Offer treasure if necessary.
I'm Calling In My Favor: Works in town. An NPC they owe asks them to pay back that debt, by dealing with some issue in an already-developed area. If it gets them more ready for that blank space in the process, so much the better . . .
Some of these are pretty heavy handed, but all of them at least keep you playing, and add to the fun of the dungeon, without you needing to put up an "under construction" sign or forcefield without explanation or to say "Guys, can you go there another time? I didn't write it up."
Keep these to a minimum - prep is always better, and just plain seat-of-your-pants making crap up is fine if you can do it. This is for when you can't, or when you're really going to have a better game next time if you can just avoid having to wing it this game.
If you also use these kinds of encounters in your game, it won't even seem so odd. "Another of those magic key doors? Damn, this Mad Wizard must have had a stake in the Locksmith's Guild." Some of them can be used in a very heavy handed way if the dungeon is ruled by an Adversary (Undermountain springs to mind). If the Mad Wizard is still down there, it's possible he could send up an illusionary image of himself and a D&D Wall of Force / GURPS Force Dome and say "Turn back, fools, this way is not for you . . . yet!"
I'm sure some well-prepared GMs never have this happen. Well, I'm moderately well prepared, but I don't have the time or resources to develop everything before I let my players loose. My game is an ongoing beta test in that way - sometimes, you can't do that yet even though I've promised you will be able to at some point. I don't apologize for this, because I have even less time for that.
Use these in good fun.