In a WebmasterWorld thread, a user who maintains multiple Google accounts is running into problems because when he is logged in to one user account, Google apparently randomly switches to another account.
A number of users report that this is an issue related to Firefox's Password Manager. If you store multiple passwords for Google accounts in your Firefox Password Manager, it seems that Firefox switches accounts without user input.
One user has worked around this problem with a Firefox addon called CookieSwap. However, that is a less than ideal solution.
Forum discussion at WebmasterWorld.