Change summary to reflect the real issue.

Here's the stacktrace of the clear action in question:

Stack trace:
#0 [internal function]: Cite->clearState(Object(Parser))
#1 phase3/includes/Hooks.php(216): call_user_func_array(Array, Array)
#2 phase3/includes/GlobalFunctions.php(3734): Hooks::run('ParserClearStat...', Array)
#3 phase3/includes/parser/Parser.php(302): wfRunHooks('ParserClearStat...', Array)
#4 phase3/includes/parser/Parser.php(4508): Parser->clearState()
#5 phase3/includes/parser/Parser.php(499): Parser->startParse(Object(Title), Object(ParserOptions), 3, true)
#6 phase3/includes/parser/Parser.php(4539): Parser->preprocess('???????????????...', Object(Title), Object(ParserOptions))
#7 phase3/includes/cache/MessageCache.php(794): Parser->transformMsg('???????????????...', Object(ParserOptions), Object(Title))
#8 phase3/includes/Message.php(467): MessageCache->transform('???????????????...', false, Object(LanguageZh), Object(Title))
#9 phase3/includes/Message.php(303): Message->transformText('???????????????...')
#10 phase3/includes/Message.php(340): Message->toString()
#11 phase3/includes/OutputPage.php(772): Message->text()
#12 phase3/includes/OutputPage.php(816): OutputPage->setHTMLTitle(Object(Message))
#13 phase3/includes/SpecialPage.php(546): OutputPage->setPageTitle('???????????????...')
#14 phase3/includes/specials/SpecialPrefixindex.php(43): SpecialPage->setHeaders()
#15 phase3/includes/SpecialPageFactory.php(475): SpecialPrefixindex->execute('M')
#16 phase3/includes/SpecialPageFactory.php(514): SpecialPageFactory::executePath(Object(Title), Object(RequestContext), true)
#17 phase3/includes/parser/Parser.php(3245): SpecialPageFactory::capturePath(Object(Title), Object(RequestContext))
#18 phase3/includes/parser/Preprocessor_DOM.php(1043): Parser->braceSubstitution(Array, Object(PPFrame_DOM))
#19 phase3/includes/parser/Parser.php(2950): PPFrame_DOM->expand(Object(PPNode_DOM), 0)
#20 phase3/includes/parser/Parser.php(1068): Parser->replaceVariables('<ref>a</ref>?<!...')
#21 phase3/includes/parser/Parser.php(345): Parser->internalParse('<ref>a</ref>?<!...')
#22 phase3/includes/WikiPage.php(2836): Parser->parse('<ref>a</ref>?<!...', Object(Title), Object(ParserOptions), true, true, 75)
#23 phase3/includes/PoolCounter.php(187): PoolWorkArticleView->doWork()
#24 phase3/includes/Article.php(556): PoolCounterWork->execute()
#25 phase3/includes/Wiki.php(495): Article->view()
#26 phase3/includes/Wiki.php(266): MediaWiki->performAction(Object(Article))
#27 phase3/includes/Wiki.php(619): MediaWiki->performRequest()
#28 phase3/includes/Wiki.php(538): MediaWiki->main()
#29 phase3/index.php(58): MediaWiki->run()
#30 {main}

Maybe it was introduced when Message class was added and exists in some other special pages.