Which bit would you flip?

You are an evil computer genius. You have access to the computer belonging to your arch-nemesis. Allowing only the flipping of a single bit, in order to inflict the most damage to the contents of the computer, what bit would you flip?

(Assume that in addition to having the ability to flip arbitrary bits you also know where each important piece of data lives, eg. the MBR. Additionally, assume the error correction algorithms fail to detect this change)