Unlimited experience(The Zombie Island Of Dr. Ned)
After you have activated the three turret in Jacobs Cove, one of them will be down and not scanning the area for zombies. If you attack this turret, the shield will go down and it will take damage. If you manage to do enough damage, the turret will explode but not disappear, giving you 1,296 experience points, a random level 48 shield item, and some experience for whatever weapon you where using to attack the turret (on your second playthrough). It has a very large amount of health, but it can be done. Note: Using a weapon with acid damage is best for the increasing bonus damage. It can be repeated an indefinite number of times on your second playthrough.