C# List remove from end, really O(n)?

In general List<T>::RemoveAt is O(N) because of the need to shift elements after the index up a slot in the array. But for the specific case of removing from the end of the list no shifting is needed and it is consequently O(1)

Removing last item will actually be O(1) operation since only in this case List doesn't shift next items in array. Here is a code from Reflector:

this._size--;
if (index < this._size) // this statement is false if index equals last index in List
{
    Array.Copy(this._items, index + 1, this._items, index, this._size - index);
}
this._items[this._size] = default(T);

This should give you an idea

    public void RemoveAt(int index) {
        if ((uint)index >= (uint)_size) { 
            ThrowHelper.ThrowArgumentOutOfRangeException(); 
        }
        _size--; 
        if (index < _size) {
            Array.Copy(_items, index + 1, _items, index, _size - index);
        }
        _items[_size] = default(T); 
        _version++;
    }