I had the exact opposite thing happen to my lifting partner. He left his Vassive-NO, an optimum nutrition Arginine product, in the trunk of his car. The pills changed from a creamy light yellow to a dark yellow with brown spots. I can't say what changed but he took them anyways and he's still alive.
I had an experience during the winter where i left Hydroxycut Hardcore in my car and the capsules froze. I still had some great fat loss so the effect, if any, on the chemicals was minimal.