_paged throws an error if there is already a SIP on the current page.
#3534 / _paged throws an error if there is already a SIP on the current page.
_paged renders the the current client parameter into a return URL. If the current client already contained a SIP, a check in Sip.php threw an exception, to prevent using the parameter 's' by the user.
Sip.php: New: the exception is only thrown if the SIP is unknown.