This morning between 6:30 a.m. PST and 7:25 a.m. PST every Google search result displayed the notice "This site may harm your computer." Clicking on the link would take you to a support page effectively blocking any access to all sites in the results.
Monitoring malicious sites is pretty much part of the territory for Google, and shows they take malware on the Web seriously. Google receives a list of malicious sites from StopBadware.org that is used to flag search results that may pose threats to users. According to Mayer, the list is maintained by humans due to the research involved for each case.
"We periodically receive updates to that list and received one such update to release on the site this morning." Mayer said in the post, "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."
Google reports the errors first appeared between 6:27 a.m. and 6:40 a.m. and began disappearing between 7:10 and 7:25 a.m.
If you're interested in learning more about Google's efforts when it comes to malware, take a look at their analysis of malware in The Ghost in the Browser (PDF).
Update: The StopBadware blog talks about the 'Google Glitch'
StopBadware says that the glitch "led to a denial of service of our website, as millions of Google users attempted to visit our site for more information."
Additionally, StopBadware claims Mayer's statement about Google getting URLs from StopBadware is not accurate: "Google generates its own list of badware URLs, and no data that we generate is supposed to affect the warnings in Google's search listings. We are attempting to work with Google to clarify their statement."
Update 2: Google post updated
Google updated its post to say that StopBadware does not in fact provide a list; instead, it helps Google to "come up with criteria for maintaining this list."
Matt Cutts tweets that the error was on Google's side