The following wikitext contains a DOT syntax error:
<graphviz>
graph test {

A -> B;

}
</graphviz>

The arrow (->) from A to B is only allowed in directed graphs (DOT keyword "digraph") whereas the graph above is undirected (DOT keyword "graph").

The graphviz command reports a warning about this but the warning is not displayed by the GraphViz extension. Worse, since the graph is effectively empty, no graph image file is produced which results in an internal error:

Internal error

[488c2db8] /wdev/index.php?title=Main_Page&action=submit MWException from line 171 of /home/welterkj/public_html/mediawiki/git/core/extensions/GraphViz/UploadLocalFile.php: UploadLocalFile::processVerificationError: Unexpected value 3

Backtrace:

#0 /home/welterkj/public_html/mediawiki/git/core/extensions/GraphViz/UploadLocalFile.php(99): UploadLocalFile::processVerificationError(array, Language, string)
#1 /home/welterkj/public_html/mediawiki/git/core/extensions/GraphViz/GraphViz_body.php(1117): UploadLocalFile::isUploadAllowedForTitle(User, string, string, boolean, Language, string)
#2 /home/welterkj/public_html/mediawiki/git/core/extensions/GraphViz/GraphViz_body.php(802): GraphViz::render(string, array, Parser, PPFrame_DOM)
#3 [internal function]: GraphViz::graphvizParserHook(string, array, Parser, PPFrame_DOM)
#4 /home/welterkj/public_html/mediawiki/git/core/includes/parser/Parser.php(4209): call_user_func_array(array, array)
#5 /home/welterkj/public_html/mediawiki/git/core/includes/parser/Preprocessor_DOM.php(1252): Parser->extensionSubstitution(array, PPFrame_DOM)
#6 /home/welterkj/public_html/mediawiki/git/core/includes/parser/Parser.php(3319): PPFrame_DOM->expand(PPNode_DOM, integer)
#7 /home/welterkj/public_html/mediawiki/git/core/includes/parser/Parser.php(1228): Parser->replaceVariables(string)
#8 /home/welterkj/public_html/mediawiki/git/core/includes/parser/Parser.php(431): Parser->internalParse(string)
#9 /home/welterkj/public_html/mediawiki/git/core/includes/content/WikitextContent.php(333): Parser->parse(string, Title, ParserOptions, boolean, boolean, NULL)
#10 /home/welterkj/public_html/mediawiki/git/core/includes/content/AbstractContent.php(497): WikitextContent->fillParserOutput(Title, NULL, ParserOptions, boolean, ParserOutput)
#11 /home/welterkj/public_html/mediawiki/git/core/includes/EditPage.php(3522): AbstractContent->getParserOutput(Title, NULL, ParserOptions)
#12 /home/welterkj/public_html/mediawiki/git/core/includes/EditPage.php(2320): EditPage->getPreviewText()
#13 /home/welterkj/public_html/mediawiki/git/core/includes/EditPage.php(560): EditPage->showEditForm()
#14 /home/welterkj/public_html/mediawiki/git/core/includes/actions/EditAction.php(56): EditPage->edit()
#15 /home/welterkj/public_html/mediawiki/git/core/includes/actions/SubmitAction.php(40): EditAction->show()
#16 /home/welterkj/public_html/mediawiki/git/core/includes/MediaWiki.php(403): SubmitAction->show()
#17 /home/welterkj/public_html/mediawiki/git/core/includes/MediaWiki.php(275): MediaWiki->performAction(Article, Title)
#18 /home/welterkj/public_html/mediawiki/git/core/includes/MediaWiki.php(571): MediaWiki->performRequest()
#19 /home/welterkj/public_html/mediawiki/git/core/includes/MediaWiki.php(422): MediaWiki->main()
#20 /home/welterkj/public_html/mediawiki/git/core/index.php(46): MediaWiki->run()
#21 {main}