C# foreach loop - is order *stability* guaranteed?

Suppose I have a given collection. Without ever changing the collection in any way, I loop through its contents twice with a foreach. Barring cosmic rays and what not, is it absolutely guaranteed that the order will be consistent in both loops?

Alternatively, given a HashSet<string> with a number of elements, what can cause the output from the the commented lines in the following to be unequal:

{
    var mySet = new HashSet<string>();
    // Some code which populates the HashSet<string>

    // Output1
    printContents(mySet);

    // Output2
    printContents(mySet);
}

public void printContents(HashSet<string> set) {
    foreach(var element in set) {
         Console.WriteLine(element);
    }
}

It would be helpful if I could get a general answer explaining what causes an implementation to not meet the criteria described above. Specifically, though, I am interested in Dictionary, List and arrays.