If you are using multiple Google accounts, you have to sign off before signing in to another Google account.
Google is testing a new feature that allow users to use multiple Google accounts in the same browser at the same time. This feature only works for Google Calendar, Gmail, Google Docs, Google Reader, Google Sites and Google Code. Other Google services will be added later when this feature available for public.
Remember: if you turn on this feature, Gmail’s Offline Mode will be disable automatically.