It probably won’t surprise many people that Room 6 of the Caves of Chaos in the 5e playtest materials has set some hair on end. By the book description, room 6 can have up to 40 kobolds in it. Needless to say, this has offended the sensibilities of people who believe all encounters in roll playing games should be “balanced”. What hope does a group of 5 PCs have against a horde of 40 kobolds (incidentally, the original module also had 40 kobolds in this room, 17 male, 23 female). The answer of course is: “Without very careful planning and consideration, very little.” Which is as it should be.
It seems to me that when people these days talk about “balanced” encounters, what they really mean is “weighted in such a way that the PCs have a roughly 60-75% chance of being successful, unless it’s a BBEG battle, in which case it’s more acceptable for the chances of success to be between 50-60%, and under no circumstances should it ever dip below 50% unless there is a story reason for the PCs to be defeated.” That’s not a balanced encounter. That’s a rigged encounter in favor of the PCs. If we want to talk about a balanced encounter, we would be talking about encounters where 50% of the time, the PC’s will lose. Honestly, that’s not all that fun in a game where combat is expected (if not outright encouraged), so even in the deadliest forms of early D&D, encounters were weighted such that in most of them, the PCs had a > 50% chance of wining if they weren’t actively self destructive. But older editions believed in a larger balance, one across the entire world, or dungeon, wherein while the PCs might for the sake of game have the upper hand most of the time in the right sections, sometimes you came across encounters you just couldn’t win at your level, or with your skills. Sometimes you retreat and try again. There’s nothing wrong with this.
And if the PCs decide to rush headlong into the room of 40 kobolds without preparation. Well, then honestly, they deserve everything that’s coming to them.
Now there is one problem with the 40 kobold room as it relates to the current designs of D&D games. You only get experience for killing or otherwise defeating monsters. In old D&D, the bulk of your XP came from treasure, which honestly I think they should bring back.