Never had an issue. The melting point of aluminum (and lexan) is way higher than 212 but the plastic is not. The lid strap on the Nalgene is not removable. With true frostbitten toes, as I'm sure you know, re-warming is only done in the hospital prior to amputating the non-viable toes after demarcation. Your point is well taken about refreezing and why mountaineers don't rewarm on the hill.