The second time I used my 15gal Rubbermaid cooler I heard a lot of cracking noises. But noticed nothing when I took the grain out. I think the inside insulation was cracking. The walls didn't warp or anything. I've used it a few times since then.
Primary: Bipolar Squirrel Nut Brown, No Oats Oatmeal Stout
Bottled: Scottish 60/, Aunt Gingers XMas Ale, No Play I.P.A.
Kegged: Still dreaming.
"I have a right to nothing which another has a right to take away." - Thomas Jefferson