Nobody likes a railroad, least of all proponents of the OSR. Apparently there has been some debate about the evil of the Quantum Ogre—an encounter that gets dropped in front of the players regardless of where they go or what they do.
An adventure involving the quantum ogre is bad because the players’ choices don’t matter: either they don’t have enough info to make a meaningful choice or the information they have is useless since the quantum ogre will show up no matter what they do. They have no agency – they have no capacity “to make choices and to impose those choices on the world.” Either they cannot make a meaningful choice because they lack information, or they cannot impose their choice on the world because the quantum ogre shows up anyway.
This is a formulation of why I really, really dislike plotting adventures. Apart from plotting being too damned much work, it creates what I feel is an inauthentic experience for the players. That lack of agency turns into frustration, that frustration turns (at best) into an attempt to regain agency, which causes a problem for me: I’m not prepared to do anything outside the plot and the improvised part of play comes off much flatter than the plotted part. It’s a dynamic I really don’t enjoy, so I’ve been striving for the last few years to figure out how to avoid plotting, even when I don’t have a lot of time to prep.
Alex also mentions the apparent distinction between the Quantum Ogre and random encounters, the latter being a favourite staple of the Old School even while the former is detested as illusionist play. His response is that random encounters are slightly better, because the GM is forced to improvise how to work in this encounter.
I think it’s a bigger difference than that.
The essential disconnect I see between fans of random encounters and their detractors is that I don’t see the random encounter roll as the beginning of a scene  where the PCs face the creature, but the beginning of their awareness that something is out there. There are then two parts to a random encounter: the opportunity to notice information and investigate, and the face-off itself. This opportunity is crucial to making a random encounter not just another quantum encounter. By having a chance to engage with hints and clues about the existence of a threat or opportunity , the players can make informed choices about this particular encounter. They may choose to confront it, escape it, stalk it until they have the advantage, or otherwise deal with it more-or-less intelligently.
Choice is the ingredient that gives players agency and keeps a game from being a railroad. Random encounters are no different.
What about surprise?
Of course, sometimes, as the DM, you’re going to turn the screws a bit and ask a different question: instead of “Do you want to deal with this thing? How?” you might ask “You’re already faced with this thing! What are you going to do about it? Run? Fight? Door number three?” That’s totally legitimate, but carries with it the whiff of railroading and opens the door to the same frustrations as the Quantum Ogre.
The old school has an answer for this, and it’s a parallel to the random encounter itself: the DM can disclaim fiat choice and turn to the dice. There’s a random encounter, and the DM doesn’t know whether a direct encounter or a distant, clue-laden approach is best for play and the player’s mood right now. Take it out of the DM’s hands and put the question to the dice: Roll for surprise!
Randomness is another tool to avoid railroads. The DM gets put into the same position as the players when they disclaim choice and trust the dice, in that they are equally as surprised by what happens next as the players are. Rather than DM tyranny and imposed ideas, the twist is left to fate.
If the DM isn’t forcing the encounter but the dice say it happens, it may not be be obvious to the players whether this is an instance of a Quantum Encounter or random chance. However, as the encounters add up over the hours and over the sessions, players can tell the difference. The Quantum Ogre is a problem precisely because players can tell over time that the DM’s hand is laying heavy on the encounters they face. By the same mechanism of social insight, players can also tell when that heavy hand is missing, and the randomness of the dice is one of the best ways of taking the DM’s hand off.
Alex’s post also links to a whole lot of other posts discussing the problem and solutions to the Quantum Ogre and is well worth your visit.
“Scene”, only for lack of a better word. I don’t tend to think in terms of scenes when I run old-school games.
Not all random encounters are monsters intent on the PC’s death! Just as often they’re neutral or positive opportunities. It all turns on what the PCs and DM together decide to do with the encounter. Even a clearly-hostile creature can be turned into an opportunity: try having them approach, but not immediately launch into a to-the-death fight. See what happens when the players are faced with a potential enemy who doesn’t immediately attack them.