You can't end the year overspending—that's what the issue is. It has happened very rarely, and when it's happened it has not been a good time for the department or agency that did it. If you can get supplementaries, that may be all right, but we live in a world where things can change quickly. In years in which there's talk of elections and things, you don't know if you'll be able to cut a supplementary. Moreover, if you start the year off planning to spend more than Parliament has actually voted to you, it doesn't sound like you're being very transparent with parliamentarians.
On June 1st, 2009. See this statement in context.