When this happens, a 'Can\'t set headers after they are sent.' error often follows if the response was already started.
A few example pages where this happens from the production logs:
eswiki:Usuario:Alberperez
enwiki:User_talk:Cylon
enwiki:User_talk:FunkeeMonkey12
enwiki:User:Rlloyd3
enwiki:User_talk:Nskahill
Version: unspecified
Severity: normal