The stagecoach is being pursued by the men who want to kill Audra, and the water in the well had been salted at the relay station. They all go into the relay station to spend the night and mention that there is a barrel of water on the stagecoach. So why didn't anyone think about bringing it inside to prevent it from being spilled out by the outlaw?
Hmm, from what I recall, as I saw it just recently, is that it wasn't spilled out by the outlaw. What little was left, was stolen by Mr. Lassiter while he was on watch. He took it and then rode off on one of the horses.
But given how fidgety Lassiter was, I agree with you. I wouldn't have trusted him to be on watch, and I would have insisted that the barrel be brought inside. Then again, I guess we wouldn't have had one of the most compelling episodes regarding the human condition if that had happened. :)
Just looked at it again. The one barrel of water on the back of the wagon was all the water they had until they reached the station where they thought they would be able to get more water from the well, which ended up being salted. They rationed the water in the barrel because that was all of the drinking water that they had. The well water was tampered with but not the water in the barrel that was attached to the rear of the wagon.
What you might be referring to is that one of the horses caught a pebble under its shoe, thus forcing the driver to leave it behind. This made their trip longer which is also why they had to ration the water. They deduce later that the horse "picking up a pebble" was likely no accident.
Anyway, while on watch, Lassiter sneaks off, fills up his canteen with what little water was left in the barrel, takes one of the horses, and rides off.