Many current browsers allow you to surf different pages simultaneously through separate tabs. Adobe Analytics stores a cookie in the browser, which is immediately shared across all tabs and windows. Any user who browses your site in multiple tabs is considered a single visit.
The time an image request is fired determines a user's path. If a user travels around your site in several tabs, pathing reports can show subsequent pages where links technically don't exist.
Browsing a site in multiple tabs still follows standard visit expiration (12 hours of activity or 30 minutes of inactivity from any tab or window). Browsing a site in separate browsers, however, is treated as separate visits and visitors. Since separate browsers use separate cookie storage locations, they are considered completely independent.