When session isn't started and Tracy with user panel and tab (Bridges/SecurityTracy/templates/UserPanel*.phtml) is used, nette exception (or php warning) 'Cannot send header after HTTP headers have been sent' is presented.
It's because of both templates directly check (e.g. there) nette user which tries to start session.
Suggested Solution: to check if session is started before nette user/identity is checked - e.g.
<?php if (session_id() && $user->isLoggedIn()): ?>Logged in<?php else: ?>Unlogged<?php endif ?>
Described problem is presented in both templates - UserPanel.panel.phtml, UserPanel.tab.phtml