The Gemoro on Shevuos 21b describes a situation where a person including permitted and forbidden items in the same Shevuo. Rashi explains that by merely forbidding all foods, he is implicitly including forbidden foods.
However, on 23b, the Gemoro says the same thing, and Rashi says that the case is where one explicitly states that he is forbidding both permitted and forbidden items.
Admittedly, the second Gemoro is talking about a case of Mefaresh, and maybe the first Rashi is just telling us that in the general case, one need only forbid all foods (without mentioning forbidden foods); but I wonder if there is more to be said here.
Rashi on 23b is explaining -- according to the Havah Amina -- that Ma'achalim Asurim are included in Ochlim She'einam Re'uyim, and therefore are excluded in the case of the Reisha. However, in the Maskana, we say that Ma'achalim Asurim are Ochlim ha'Re'uyim, and therefore the Reisha already reads that if someone made a Shevu'ah not to eat he may not eat Isurim, like Rashi says on 21b.