If you had been using Microsoft's OneCare and Google's Gmail over the weekend(the browser portion), you, like many others, would have been surprised to see that the two weren't getting along. While corrected now, OneCare was identifying the Gmail site as being virus infected. Within hours of the initial update, concerned people were posting on Microsoft's forums to find out more. The flaw was admitted, and quickly fixed:

The software giant has apologised for the false positive alert on the popular webmail service, which it attributes to mistakes in understanding coding changes made to Gmail last week. The software giant is reviewing its procedures to prevent a repetition of the problem.
False positives are quite common, and while sometimes a problem, are typically just minor annoyances. If you are using OneCare, you should already have the amended update. Microsoft stated they'd try to prevent the problem from repeating, but in truth with the billions of combinations of code that can occur, all AV developers will suffer this from time to time.