Just out of curiosity, has anyone had the inside of their Rubbermaid warp? Mind warped after the very first mash. It isn't causing a problem, but there is now a pretty large lump that starts at the bottom and goes about 2/3 of the way up the side getting smaller as it goes. Not big deal, but it did make me question the quality of the cooler I got, especially because it was a Home Depot labeled Rubbermaid and I know they make people build crap parts for them to sell cheaper.
I have the Home Depot branded one too. I haven't brewed with it yet, but I did fill it with hot (~155F) water after I put together the plumbing, and didn't get any warping.
I wonder if maybe you got some hot water between the inner and outer walls of the cooler...? Is the warp close to the side with the spigot?