Running buddypress? Are you getting a debug notice like this:
The current user is being initialized without using $wp->init()
and you cannot figure out where it is coming from ?
Most likely one of your plugins is calling a user related wordpress function a bit early (see wordpress action sequence). You could try deactivating them until the debug message goes away. The last plugin deactivated will be it. Let the developer know because his/her code may not be quite working the way it was intended to.
These need to be used after the init action. The plugin or theme might have something like
add_action ('plugins_loaded', 'some_function');
The action should be added later, when the current user has been initialised.
add_action ('init', 'some_function');