Surprise is one of those rules that I often see misapplied or simply done wrong. I admit I've not always done it correctly in the heat of the moment, but I honestly don't know why some people have trouble with it Perhaps it's because of its actual simplicity. In any case, let's take a look at it, shall we?
Like many rules in games like BX, the surprise mechanic is fairly abstract and is used to cover a lot of different scenarios. Being surprised is not just standing agog at the fact that there are six goblins with spears in front of you. Neither is it necessarily a "jump scare" type ambush.
Being surprised can be as simple as just being caught flat-footed; or a moment's inattention that gave the opposition the chance to act first. Maybe the door took an extra shove to open or the floor was a bit uneven. The point is you're playing a bit of catch-up for a few seconds.
Once the DM makes the call that surprise is at least possible for one or both groups, I think there are two main areas that throw some people off mechanically: The order of events and the nature of the dice rolls themselves.
Surprise isn't rolled until step 5 of the turn order. By that time, the party has already entered the area. Narratively speaking, people would expect to hear about the encounter right at that point. The thing to remember is that all this is more or less happening at once.
"If surprise is possible, the DM should roll 1d6 for each side in the encounter. A result of 1 or 2 for either side indicates that the side is surprised (unless given otherwise in the monster description)."The stumbling block here seem to be that the die roll for "your" side is actually based on your opponent, not you (the PCs). It's the chance you are surprised, not of you doing the surprising. This becomes apparent when dealing with creatures whose surprise roll is not the default 1-2. For example, a bugbear surprises on a 1-3. That means the chance of PCs being surprised goes up.I've had to remind people of this at the table more than once.
I'd like to point out that surprise can actually be a big help to PCs. With smart play, the PCs will have many opportunities to be the surprise as opposed to the surprised. Things like sneaking thieves reconnoitering, careful listening, and detection spells give a party a decent to get the drop on things. It's important to pay attention to the text on B23. It points out that surprise is not always possible, but that doesn't mean it can't be possible for one side and not the other. Honestly, in my experience, PCs being surprised actually doesn't come up all that often.
Despite this, I frequently hear complaints from players that surprise is just for screwing the PCs over: "How can we be surprised by the goblins?! We're in a dungeon! We're ALWAYS expecting an attack!" and so forth.
If this sounds familiar, perhaps consistently applying these rules in a way that doesn't slow down play can smooth things out a bit and let you enjoy the added richness they can give to your encounters.