Found while testing fix for T127860: MobileFrontend using ResourceLoaderGetConfigVars hook with User dependent data locally:
2016-02-23 20:21:48 wiki exception ERROR: [911d102a] /w/load.php?debug=false&lang=en&modules=startup&only=scripts&skin=vector BadMethodCallException from line 960 of /vagrant/mediawiki/includes/session/SessionManager.php: Sessions are disabled for this entry point {"exception_id":"911d102a"}
[Exception BadMethodCallException] (/vagrant/mediawiki/includes/session/SessionManager.php:960) Sessions are disabled for this entry point
#0 /vagrant/mediawiki/includes/session/SessionManager.php(187): MediaWiki\Session\SessionManager->getSessionFromInfo(MediaWiki\Session\SessionInfo, WebRequest)
#1 /vagrant/mediawiki/includes/WebRequest.php(664): MediaWiki\Session\SessionManager->getSessionForRequest(WebRequest)
#2 /vagrant/mediawiki/includes/user/User.php(1175): WebRequest->getSession()
#3 /vagrant/mediawiki/includes/user/User.php(384): User->loadFromSession()
#4 /vagrant/mediawiki/includes/user/User.php(5093): User->load()
#5 /vagrant/mediawiki/includes/user/User.php(2706): User->loadOptions()
#6 /vagrant/mediawiki/includes/context/RequestContext.php(368): User->getOption(string)
#7 /vagrant/mediawiki/includes/StubObject.php(204): RequestContext->getLanguage()
#8 /vagrant/mediawiki/includes/StubObject.php(160): StubUserLang->_newObject()
#9 /vagrant/mediawiki/includes/StubObject.php(104): StubObject->_unstub(string, integer)
#10 /vagrant/mediawiki/includes/StubObject.php(179): StubObject->_call(string, array)
#11 /vagrant/mediawiki/includes/cache/MessageCache.php(824): StubUserLang->__call(string, array)
#12 /vagrant/mediawiki/includes/cache/MessageCache.php(767): MessageCache->getMessageFromFallbackChain(StubUserLang, string, boolean)
#13 /vagrant/mediawiki/includes/Message.php(1148): MessageCache->get(string, boolean, StubUserLang)
#14 /vagrant/mediawiki/includes/Message.php(771): Message->fetchMessage()
#15 /vagrant/mediawiki/includes/Message.php(874): Message->toString()
#16 /vagrant/mediawiki/extensions/WikimediaMessages/WikimediaMessages.hooks.php(137): Message->plain()
#17 /vagrant/mediawiki/includes/Hooks.php(195): WikimediaMessagesHooks::onMobileLicenseLink(string, string, array, string)
#18 /vagrant/mediawiki/extensions/MobileFrontend/includes/MobileFrontend.skin.hooks.php(100): Hooks::run(string, array)
#19 /vagrant/mediawiki/extensions/MobileFrontend/includes/MobileFrontend.hooks.php(436): MobileFrontendSkinHooks::getLicense(string)
#20 /vagrant/mediawiki/includes/Hooks.php(195): MobileFrontendHooks::onResourceLoaderGetConfigVars(array)
#21 /vagrant/mediawiki/includes/resourceloader/ResourceLoaderStartUpModule.php(109): Hooks::run(string, array)
#22 /vagrant/mediawiki/includes/resourceloader/ResourceLoaderStartUpModule.php(370): ResourceLoaderStartUpModule->getConfigSettings(DerivativeResourceLoaderContext)
#23 /vagrant/mediawiki/includes/resourceloader/ResourceLoaderModule.php(707): ResourceLoaderStartUpModule->getDefinitionSummary(DerivativeResourceLoaderContext)
#24 /vagrant/mediawiki/includes/resourceloader/ResourceLoader.php(622): ResourceLoaderModule->getVersionHash(DerivativeResourceLoaderContext)
#25 [internal function]: Closure$ResourceLoader::getCombinedVersion(string)
#26 /vagrant/mediawiki/includes/resourceloader/ResourceLoader.php(623): array_map(Closure$ResourceLoader::getCombinedVersion;1299315079, array)
#27 /vagrant/mediawiki/includes/resourceloader/ResourceLoader.php(675): ResourceLoader->getCombinedVersion(ResourceLoaderContext, array)
#28 /vagrant/mediawiki/load.php(43): ResourceLoader->respond(ResourceLoaderContext)
#29 /var/www/w/load.php(5): include(string)
#30 {main}$wgUser is unstubbed due to the wfMessage() call in WikimediaMessagesHooks::onMobileLicenseLink() which does not specify a content language. Since this is done inside the ResourceLoaderGetConfigVars hook it should not vary on user.