The VB6-styled "Collection" object allows for modification during enumeration, and seems to work sensibly when such modifications occur. Too bad it has other limitations (key type is limited to case-insensitive strings) and doesn't support generics, since none of the other collection types allow modification.
Frankly, I'm not clear why Microsoft's iEnumerable contract requires that an exception be thrown if a collection is modified. I would understand a requirement that an exception be thrown if a changes to a collection would make it impossible for an enumeration to continue without wackiness (skipping or duplicating values that did not change during enumeration, crashing, etc.) but see no reason not to allow a collection that could enumerate sensibly to do so.
