How do I reach a barrel stuck on a cliff?

As in Borderlands 1, there are a few random pathing and clipping glitches that can prevent you from finding an item, completing a mission, or even moving. Most can be corrected by simply restarting the game.

Usually choosing exit from the pause menu and then continuing will reset your location to the nearest quick transit terminal, reset the zone or area you're in, and reset the mission you're on. You might loose a bit of progress but not much.

However, there there is a small potential for one of these glitches to occur in such a way that the glitch gets preserved if you choose the exit option (which saves your current progress). If this occurs the only way to recover is to start the game over from the beginning.

To prevent this from happening, anytime I play Borderlands (or any other semi-randomized or procedurally made game) I always do a soft restart whenever I encounter a glitch. I might loose some progress, exp, weapons, etc, but it guarantees that I can recover from the glitch without having to start the game over again.

On the Xbox 360 hit the Xbox button on the controller and choose return to console window; or just power off the Xbox and turn it back on. On PC force-close the application by alt-tabbing out of it, opening up task manager, and killing the process.

From the top of my head the only solution I see right now is to restart your game and redo the mission. The truck will still be here and you'll be able to shoot it to get the barrels you're missing.