From Paul Kay, this passage from an email recently sent by the Chair of the UC Berkeley Academic Senate:
As background, the state continues to anticipate a very large deficit this year. While the Governor's budget did call for restoring approximately $300M in funds cut last year from the UC budget, it would probably not be prudent - in my view - to assume that our budgetary situation for 2010-11 will be better than 2009-10. In addition, the Governor's proposed constitutional amendment, which called for shifting money to higher education by privatizing the state prison system, has attracted no political support (and a great deal of criticism as a policy proposal). While UCOP has been apparently working with to develop alternative constitutional amendments, it would again be imprudent not to expect a silver bullet.
As Paul points out, the missing words between with and to ("the Governor's Office"?) suggests that this note was sent in haste, so that the overnegation in the last phrase may be an editing error.
For example, the author might have begun with "it would be prudent not to expect a silver bullet", and then decided to change it to "it would be imprudent to expect a silver bullet", but made only one of the two required changes.
But there's also something interesting going on with the "silver bullet" metaphor. Even at Berkeley, I doubt that the author really meant to bring up witches and werewolves. The Lone Ranger doesn't quite fit, either. But as the Wikipedia article explains,
The term has been adopted into a general metaphor, where "silver bullet" refers to any straightforward solution perceived to have extreme effectiveness. The phrase typically appears with an expectation that some new technology or practice will easily cure a major prevailing problem.
Maybe there's been a merger with Paul Ehrlich's "magic bullet" phrase. Anyhow, a quick web search shows that there's a lot of talk going around about expected silver bullets, little or none of it from evil supernatural creatures.
[Update — the OED explicitly suggests that the sense "A simple, miraculous solution to a complex and difficult problem" is related to the medical magic bullet phrase, and gives citations back to 1951:
1951 Bedford (Pa.) Gaz. 19 Sept. 1/3 There are those who warn against viewing the atom as a magic weapon… I agree. This is not a silver bullet which can deliver itself or otherwise work military miracles. 1971 Chron.-Telegram (Elyria, Ohio) 18 Mar. 18/4 Drug abuse, as virtually every other major problem, is multicausative and not given to simplistic silver bullet solutions.
I guess it was the misnegation that set me off in the wrong interpretive direction, with a flash of the UC Berkeley Faculty Senate as a gathering of witches and werewolves being warned that "it would be imprudent not to expect a silver bullet".]