I am using MW 1.24. I am using this wikitext:
<sgallery> IMG_20140510_133239.jpg IMG_20140510_133227.jpg IMG_20140510_133225.jpg IMG_20140510_133223.jpg </sgallery>
Results:
[86f71766] /wiki/Project:Sandbox Exception from line 77 of /home/stauffenbergssh/theshatteredpan.org/w/includes/parser/StripState.php: Invalid marker: UNIQ3e909c67eef36a9c-sgallery-00000000-QINU Backtrace: #0 /home/stauffenbergssh/theshatteredpan.org/w/includes/parser/StripState.php(66): StripState->addItem(string, string, string) #1 /home/stauffenbergssh/theshatteredpan.org/w/includes/parser/Parser.php(4068): StripState->addGeneral(string, string) #2 /home/stauffenbergssh/theshatteredpan.org/w/includes/parser/Preprocessor_DOM.php(1181): Parser->extensionSubstitution(array, PPFrame_DOM) #3 /home/stauffenbergssh/theshatteredpan.org/w/includes/parser/Parser.php(3159): PPFrame_DOM->expand(PPNode_DOM, integer) #4 /home/stauffenbergssh/theshatteredpan.org/w/includes/parser/Parser.php(1222): Parser->replaceVariables(string) #5 /home/stauffenbergssh/theshatteredpan.org/w/includes/parser/Parser.php(395): Parser->internalParse(string) #6 /home/stauffenbergssh/theshatteredpan.org/w/includes/content/WikitextContent.php(335): Parser->parse(string, Title, ParserOptions, boolean, boolean, integer) #7 /home/stauffenbergssh/theshatteredpan.org/w/includes/WikiPage.php(3613): WikitextContent->getParserOutput(Title, integer, ParserOptions) #8 /home/stauffenbergssh/theshatteredpan.org/w/includes/poolcounter/PoolCounterWork.php(116): PoolWorkArticleView->doWork() #9 /home/stauffenbergssh/theshatteredpan.org/w/includes/Article.php(698): PoolCounterWork->execute() #10 /home/stauffenbergssh/theshatteredpan.org/w/includes/actions/ViewAction.php(44): Article->view() #11 /home/stauffenbergssh/theshatteredpan.org/w/includes/Wiki.php(428): ViewAction->show() #12 /home/stauffenbergssh/theshatteredpan.org/w/includes/Wiki.php(292): MediaWiki->performAction(Article, Title) #13 /home/stauffenbergssh/theshatteredpan.org/w/includes/Wiki.php(596): MediaWiki->performRequest() #14 /home/stauffenbergssh/theshatteredpan.org/w/includes/Wiki.php(448): MediaWiki->main() #15 /home/stauffenbergssh/theshatteredpan.org/w/index.php(46): MediaWiki->run() #16 {main}
It reminds me of this bug. Perhaps SmoothGallery stopped working with a certain version of MediaWiki, as MathJax did.