In addition to diet soda, as /u/jenjen1352 said, I'd say probably anything labelled as "diet" : when it replaces something that naturally has fat, it gets replaced by sugar (which the body actually transforms into fat anyway), and when it's sugar, it gets replaced by a chemical substitute that is calorie free (or very low in calories) which at best tells your system to prepare to get a lot of sugar and to transform it to fat, but then it doesn't get any and ends up eating through your muscles and neurones (a.k.a. brains); and at worst has its very own negative effects on your body, like causing cancer and whatnot. And the mildest replaces salt with other spices, which is certainly better for your health than these other substitutions, but the ones which simply removes salt without replacing it with anything tastes bland (with little effect on your health, but still better than nothing).
A better idea is to choose food and snack which is naturally better for your health (like raw fruits and veggies) and to make your own cooking, so you don't have to fill it with unhealthy stuff for it to remain beautiful (or sellable) for an unnaturally long time. Then, when you've done this, you won't feel so guilty by eating an unhealthy snack once in a while, and it won't have such a bad effect on your general health.
In addition to diet soda, as /u/jenjen1352 said, I'd say probably anything labelled as "diet" : when it replaces something that naturally has fat, it gets replaced by sugar (which the body actually transforms into fat anyway), and when it's sugar, it gets replaced by a chemical substitute that is calorie free (or very low in calories) which at best tells your system to prepare to get a lot of sugar and to transform it to fat, but then it doesn't get any and ends up eating through your muscles and neurones (a.k.a. brains); and at worst has its very own negative effects on your body, like causing cancer and whatnot. And the mildest replaces salt with other spices, which is certainly better for your health than these other substitutions, but the ones which simply removes salt without replacing it with anything tastes bland (with little effect on your health, but still better than nothing).
A better idea is to choose food and snack which is naturally better for your health (like raw fruits and veggies) and to make your own cooking, so you don't have to fill it with unhealthy stuff for it to remain beautiful (or sellable) for an unnaturally long time. Then, when you've done this, you won't feel so guilty by eating an unhealthy snack once in a while, and it won't have such a bad effect on your general health.
Edit: more natural phrasing.