So I realize that they were eager to go back to the party, and the person responsible for the barrel management said they were empty, but surely they must have felt that they were obviously not empty when they contained big, heavy dwarves? How did they not check inside?
I thought for the whole time that it was Bilbo himself who would roll them down one by one into the hatch after opening the gate. I didn't expect that the elves would be doing this without noticing.
Also, I know that elves and everything in Middle-earth wasn't "fully fleshed out" when this first book was written, but I think those elves acted a bit non-elf-like. It seems to me that elves are extremely careful and eager to do things correctly and according to protocol and would never just leave suspicious barrels like that, while they had prisoners, just to rush back to the party and take the guard's word for it even though he was obviously asleep from having drank so much, or such strong wine.
It seems like a plot hole in this otherwise very good book.