I have never heard of this before until you had brought it up. I wonder what it is exactly about diet soda that could cause a difference in a mixed drink, compared to regular soda? Anyways, diet soda usually contains aspartame, an artificial sweetener that when it reaches high temperatures, forms formaldehyde (the same chemical found in rat poison). In fact, researchers have found that in a study on mice, aspartame led to the formation of cancer. Thankfully, many soda companies are switching from aspartame to other artificial sweeteners in diet sodas, however, who is to say that those are any better? Also, many soda companies continue to use aspartame in diet sodas. The best way to find out is to simply read the ingredients on the label.