UPDATE 2: The error turns out to be the human kind (via GeekBrief.tv):
We periodically receive updates to that list and received one such update to release on the site this morning. Unfortunately (and here’s the human error), the URL of ‘/’ was mistakenly checked in as a value to the file and ‘/’ expands to all URLs. Fortunately, our on-call site reliability team found the problem quickly and reverted the file. Since we push these updates in a staggered and rolling fashion, the errors began appearing between 6:27 a.m. and 6:40 a.m. and began disappearing between 7:10 and 7:25 a.m., so the duration of the problem for any particular user was approximately 40 minutes.
UPDATE: Looks like it's fixed. Anyone still seeing the problem?
Looks like Google Search may be having some trouble this morning! As I write this, every search on Google, including searches for Google, get flagged as "may harm your computer" and if you click the link, you get an intercept page with an even bigger warning.
I'm guessing the engineers in Mountain View are on it, so enjoy the fun-making while it lasts!
NOTE: It only seems to be broken (for me!) if you're not logged in (i.e. not logged into Gmail or any other Google service). If you're logged in, Google search returns results normally.
[Thanks Antony for the tip!]