"One of these days, the dire dice of destiny are gonna land on you with both sixes down!" -Wormy
Please forgive the brief foray into a "Let me tell you about my campaign" tangent.
A couple weeks ago, a PC died fighting 3 trolls. Not surprising since he was only 4th level, but certainly not how the party wanted things to turn out. Six of the creatures attacked as the party was camped out in the Karameikan foothills making a wide loop around to approach Xitaqa from the north. Three of the trolls were dropped and burned, the remaining three surrounded and tore the fighter apart as the rest of the PCs fled.
The sad part was that the death was completely unnecessary. Not because of bad rolls or poor tactics or not having posted a sentry, but because the trolls were not part of the adventure at all. They were a random, wandering encounter.
Wandering monster rules can be a polarizing concept in D&D. Some love them, some hate them, but nearly everyone seems to have an opinion about them. For the purposes of this post, I'm referring to the Moldvay/Cook Basic & Expert (B/X) version of the rules. In a dungeon environment, there is a 1 in 6 chance of a random encounter every other turn, or every 20 minutes. In the wilderness, encounters are rolled for 1-4 times per day, determined by the GM. As a default, I tend to roll once for the daytime and once for the night. If the party is moving very slowly, or through a more densely populated area, I may increase the frequency slightly. While the chance of the encounter is less than 17%, the point is that the more time the characters take to get where they are going, the more likely they are to run into something.
Some players like this. They think it means a greater shot at experience points and possibly treasure. If nothing else, it might result in an interesting RP opportunity or a fun little fight. The truth is, it's a game of russian roulette.
A random encounter is just that, random. You never know what is going to come around the bend or through that door. While it's true that in dungeons, the charts as written are scaled somewhat for the level you are on, but even those hold a few nasty surprises. For example, Level 1 in Moldvay's charts contains the possibility (albeit slim) of encountering three different creatures with "Save or die" attacks: 1-6 Spitting Cobras, 1-6 Killer Bees, or 1-4 Crab Spiders (the numbers assume these are "wandering" and not in their lairs). Considering most XP comes from treasure in B/X, not combat, and only the Crab Spider has any loot (in its lair, not on its "person"), fighting such creatures is a low-return investment. Wilderness encounters are even worse. They are organized by terrain type, not by "power level." A party traveling through open grassland has the same chance of encountering a manticore as a merchant!
These rules may seem capricious, but I have found that they serve a very useful purpose. If one pre-supposes that the objective in a game like B/X is exploration and treasure-hunting, then it makes sense that successful parties are the ones who waste little time and minimize such encounters. Spell-casters who foolishly wastes their magic so that they need to rest sooner cost the party time. So does a fighter that needlessly pursues a combat and loses hit points that then need to be healed, thus using up a cleric's spells. Endlessly searching every square inch of wall and floor for traps or secret doors can cost hours (or more). The list goes on, but you can see the trend.
In the tangent at the start of this post. The players were "unlucky" that day because they had a daytime and a nightime encounter. Earlier in the day they had met a hill giant. Fortunately for them, it wasn't immediately hostile. Unfortunately, the attempt to negotiate passage with a gift of horseflesh resulted in an abysmal reaction roll and the fight began. The PCs dealt with a lone giant fairly quickly. The trolls were the nighttime encounter and a tough one, to be sure, but straight off the chart. It was a fair result, even if harsh. But that's life in the Old-School.
MORAL: Don't fiddle about. Know what you're goal is and stay focussed. Dragons are on some of those tables, too! I can't think of anything worse than being attacked by a red dragon flying out over you on the plains, getting half your party killed before defeating it, then having no way to track it back to its lair where the treasure (and the bulk of the XP) is!