I'm DM'ing for the first time with a group playing for the first time (Table of Noobs). In the 1st chapter of Lost Mine of Phandelver, the characters come across dead horses, are attacked by goblins, and discover a path into the woods. The campaign book says they "can easily steer the wagon away from the road and tie off the oxen while the group pursues the goblins". This, to me, implied that the wagon couldn't go down the path.
Fast forward a bit, at the end of the path is a cavern/goblin hideout, PC's enter, kill goblins and bugbear leader (Klarg), yada yada yada. In Klarg's lair, there are a bunch of supply crates, and the description says "the captured stores are bulky, and the characters will need a wagon to transport them". They have a wagon, but apparently no way to get it to the cave.
The trail is 5 miles long, so carrying the crates back to the wagon is out. My solution was to retroactively say that when they entered the clearing where the cave is, they could see another, wider path leaving the clearing that has clearly been used for moving vehicles/supplies. This was met with dissent, as it was deemed "railroading" the story and not letting them discover a way themselves.
- The dissent was from a player. Granted, there is more backstory to
the reason it was questioned (one player keeps doing things on his
own/making decisions on his own, and I "reinforced this behavior" by
giving him an out with the path. There's also a bit more chance for
argument than normal as the PC's are wife and kids :).
So finally, the related question in dealing with this problem:
Is this actually a plot hole, or have I missed something?
Another question related to my handling of this has been asked here:
Is it okay to retroactively change things when running a published adventure?
There is no time pressure
It is expected that at this point of the adventure the characters have eliminated the threats of the Cragmaw Hideout. They have defeated Klarg, and in doing so either eliminated the last of the threats there or demonstrated to any remaining goblins watching that the characters are not to be messed with, as their (unpopular) leader was just eliminated.
As such, the 5 mile trip down the trail is not really a big issue.
Anytime there is a significant lack of time pressure, and no difficulty in the task, you as the GM are free to hand wave events. I think having the characters find a second path is fine. Equally so, just saying they took the horses down the path, loaded them up, and after a few trips back and forth they managed to get all the supplies down to the wagon.
Or, just do like my group did and not worry about the details at all.
Keep the story moving to the interesting problems
The reason it seems like it is a plot hole is because the designers didn't see anything interesting in a challenge of how to get the supplies out.
They did rely on you, as the GM, to come up with some explanation if any players cared enough, but I believe they also figured the players would be happy to not micromanage their exact behavior.
So, that is the lesson I would take for both a new GM and new players: Don't micromanage how uninteresting things work, when there is no time pressure and no (ability or skill) related challenge to stand in their way.
If the players make a problem interesting, roll (or role) with it
If the players are micromanaging things, and are having fun doing so, then by all means let them have their fun.
Sometimes it is best as GM to not worry about "plot hole" like problems, and just see how the players react. If they bring up the issue, and are having fun and discussing it among themselves on what to do, let them solve their invented problem.
Perhaps they will come up with the idea of carrying the supplies down the path, in multiple trips. You can just hand wave the passage of time, instead of the actions themselves.
Or if any are looking around outside, have them roll perception and notice another path.
At the end of the day, focus on the fun. If something is uninteresting to everyone, follow my advice above and move past it.
If the players are having fun with something you originally thought would be uninteresting, then just roll with it, perhaps by throwing in some constitution checks for carrying so much, or a skill check where someone can add their land vehicle proficiency to navigating the path with the wagon (if they fail, just say they get stuck, and have to do some more (easy) checks to get out), or whatever is appropriate. Or role with it, and detail in interesting terms how the players accomplish the goal.
As long as you are having fun too (which the GM should always be having fun, as much as the players), then do whatever helps everyone have the most fun.