Having presided over exactly one complete adventure, I'm still very much a newbie as far as Dungeon Mastering is concerned. While it has some things in common with other sorts of game design-- be it board-based or electronic-- I can't really test it the way I can other designs. It's meant to be played exactly once, and if something's off-- if, say, an encounter is too easy, like my big epic boss fight that ended three rounds in without any players sustaining any damage-- then it kind of spoils the experience. There's no way for me to tell if something's not working until we break out the battle grid and start rolling some dice. (Or, story-wise, until their eyes glaze over or they respond to a serious NPC with some wisecracks.) (Or, puzzle-wise, until they miss the "clue" that I thought, wrongly, was obvious.) (Etc.)
I'm sure it's something that I'll develop a better knack for as I get more experience with it-- I'll be able to eyeball an encounter and say, yes, I better nerf this down a bit, or, alternatively, beef it up. But until I get to that point, I'm still faced with the idea that I might be sending my players into the jaws of certain doom-- certainly not something I want to happen to a group of mostly-newbies their second or third time out.
So I've built in a fail-safe to each of the encounters for my second adventure, one that I can call on should the going prove to be a little too tough-- a sort of cavalry that comes to the rescue. They're not an actual cavalry, despite the campaign's Old West flavour. What they are, precisely, I won't spoil here, in case one of my players is reading. But I will say that they're specific to each encounter, and that they're set up, in a non-obvious way, within the story of the adventure.
That's the important point, I think. If the players are getting their butts whupped, and all of the sudden, a giant dragon flies down and scoops up the baddies in its talons, it'd feel kind of cheap and obvious. But if there's a sudden influx of allies that make sense within the context of the storyline that enable the players to turn the tide, if just barely-- well, that's something exciting and epic and thus an altogether appropriate conclusion to an evening of dungeoneering, yes?