Comes up pretty often in the logs, even on action=raw, CentralNotice and api.php requests.
A few samples:
2012-10-31 22:03:32 mw43 metawiki: [752b3943] /w/index.php?title=MediaWiki:Wikiminiatlas.js&action=raw&ctype=text/javascript&smaxage=21600&maxage=86400
Exception from line 3079 of /usr/local/apache/common-local/php-1.21wmf3/includes/User.php: User::addToDatabase: hit a key conflict attempting to insert a user row, but then it doesn't exist when we select it!
#0 /usr/local/apache/common-local/php-1.21wmf3/extensions/CentralAuth/CentralAuthHooks.php(568): User->addToDatabase()
#1 /usr/local/apache/common-local/php-1.21wmf3/extensions/CentralAuth/CentralAuthHooks.php(276): CentralAuthHooks::attemptAddUser(Object(User), 'W1k1n66m')
#2 [internal function]: CentralAuthHooks::onUserLoadFromSession(Object(User), NULL)
#3 /usr/local/apache/common-local/php-1.21wmf3/includes/Hooks.php(253): call_user_func_array('CentralAuthHook...', Array)
2012-10-31 22:05:29 mw52 metawiki: [3b339d91] /wiki/Special:BannerRandom?userlang=es&sitename=Wikimedia+Commons&project=commons&anonymous=false&bucket=0&country=ES&slot=10
Exception from line 3079 of /usr/local/apache/common-local/php-1.21wmf3/includes/User.php: User::addToDatabase: hit a key conflict attempting to insert a user row, but then it doesn't exist when we select it!
#0 /usr/local/apache/common-local/php-1.21wmf3/extensions/CentralAuth/CentralAuthHooks.php(568): User->addToDatabase()
#1 /usr/local/apache/common-local/php-1.21wmf3/extensions/CentralAuth/CentralAuthHooks.php(276): CentralAuthHooks::attemptAddUser(Object(User), 'Cotillasnosotro...')
#2 [internal function]: CentralAuthHooks::onUserLoadFromSession(Object(User), NULL)
#3 /usr/local/apache/common-local/php-1.21wmf3/includes/Hooks.php(253): call_user_func_array('CentralAuthHook...', Array)
2012-10-31 22:10:43 srv203 metawiki: [453483ac] /w/index.php?title=MediaWiki:OSM.js&action=raw&ctype=text/javascript&smaxage=21600&maxage=86400
Exception from line 3079 of /usr/local/apache/common-local/php-1.21wmf3/includes/User.php: User::addToDatabase: hit a key conflict attempting to insert a user row, but then it doesn't exist when we select it!
#0 /usr/local/apache/common-local/php-1.21wmf3/extensions/CentralAuth/CentralAuthHooks.php(568): User->addToDatabase()
#1 /usr/local/apache/common-local/php-1.21wmf3/extensions/CentralAuth/CentralAuthHooks.php(276): CentralAuthHooks::attemptAddUser(Object(User), 'Aldha')
#2 [internal function]: CentralAuthHooks::onUserLoadFromSession(Object(User), NULL)
#3 /usr/local/apache/common-local/php-1.21wmf3/includes/Hooks.php(253): call_user_func_array('CentralAuthHook...', Array)
2012-10-31 22:18:02 srv301 huwiki: [7731f9cf] /w/api.php?action=query&titles...p=size&format=json
Exception from line 3072 of /usr/local/apache/common-local/php-1.21wmf2/includes/User.php: User::addToDatabase: hit a key conflict attempting to insert a user row, but then it doesn't exist when we select it!
#0 /usr/local/apache/common-local/php-1.21wmf2/extensions/CentralAuth/CentralAuthHooks.php(568): User->addToDatabase()
#1 /usr/local/apache/common-local/php-1.21wmf2/extensions/CentralAuth/CentralAuthHooks.php(276): CentralAuthHooks::attemptAddUser(Object(User), 'Blake13441')
#2 [internal function]: CentralAuthHooks::onUserLoadFromSession(Object(User), NULL)
#3 /usr/local/apache/common-local/php-1.21wmf2/includes/Hooks.php(253): call_user_func_array('CentralAuthHook...', Array)
2012-10-31 22:24:47 mw21 metawiki: [63064713] /w/index.php?title=MediaWiki:OSM.js&action=raw&ctype=text/javascript&smaxage=21600&maxage=86400
Exception from line 3079 of /usr/local/apache/common-local/php-1.21wmf3/includes/User.php: User::addToDatabase: hit a key conflict attempting to insert a user row, but then it doesn't exist when we select it!
#0 /usr/local/apache/common-local/php-1.21wmf3/extensions/CentralAuth/CentralAuthHooks.php(568): User->addToDatabase()
#1 /usr/local/apache/common-local/php-1.21wmf3/extensions/CentralAuth/CentralAuthHooks.php(276): CentralAuthHooks::attemptAddUser(Object(User), 'JorgebossGL')
#2 [internal function]: CentralAuthHooks::onUserLoadFromSession(Object(User), NULL)
#3 /usr/local/apache/common-local/php-1.21wmf3/includes/Hooks.php(253): call_user_func_array('CentralAuthHook...', Array)
2012-10-31 22:31:46 srv241 commonswiki: [36936ded] /wiki/Category:Solar_eclipse
Exception from line 3079 of /usr/local/apache/common-local/php-1.21wmf3/includes/User.php: User::addToDatabase: hit a key conflict attempting to insert a user row, but then it doesn't exist when we select it!
#0 /usr/local/apache/common-local/php-1.21wmf3/extensions/CentralAuth/CentralAuthHooks.php(568): User->addToDatabase()
#1 /usr/local/apache/common-local/php-1.21wmf3/extensions/CentralAuth/CentralAuthHooks.php(276): CentralAuthHooks::attemptAddUser(Object(User), 'Mvemjsunp8')
#2 [internal function]: CentralAuthHooks::onUserLoadFromSession(Object(User), NULL)
#3 /usr/local/apache/common-local/php-1.21wmf3/includes/Hooks.php(253): call_user_func_array('CentralAuthHook...', Array)
2012-10-31 22:36:28 mw39 metawiki: [32ca71d8] /wiki/Special:BannerRandom?userlang=da&sitename=Wikipedia&project=wikipedia&anonymous=false&bucket=0&country=DK&slot=30
Exception from line 3079 of /usr/local/apache/common-local/php-1.21wmf3/includes/User.php: User::addToDatabase: hit a key conflict attempting to insert a user row, but then it doesn't exist when we select it!
#0 /usr/local/apache/common-local/php-1.21wmf3/extensions/CentralAuth/CentralAuthHooks.php(568): User->addToDatabase()
#1 /usr/local/apache/common-local/php-1.21wmf3/extensions/CentralAuth/CentralAuthHooks.php(276): CentralAuthHooks::attemptAddUser(Object(User), 'Djbenne')
#2 [internal function]: CentralAuthHooks::onUserLoadFromSession(Object(User), NULL)
#3 /usr/local/apache/common-local/php-1.21wmf3/includes/Hooks.php(253): call_user_func_array('CentralAuthHook...', Array)
Version: master
Severity: normal