I've noticed the following error a bunch of times in our error logs. This seems to have started after our most recent upgrade (from rP8ce35e6b67e7e2a81b274bab7a6dd19dedb4df06 to rPa6698f2ea53c826c0daea7f13c354110e62ba4c6).
[29-Jul-2014 01:54:03 UTC] [2014-07-29 11:54:03] ERROR 2: htmlspecialchars(): Invalid multibyte sequence in argument at [/usr/src/libphutil/src/markup/render.php:159] [29-Jul-2014 01:54:03 UTC] #0 htmlspecialchars(string, integer, string) called at [<phutil>/src/markup/render.php:159] [29-Jul-2014 01:54:03 UTC] #1 phutil_escape_html(string) called at [<phutil>/src/markup/render.php:118] [29-Jul-2014 01:54:03 UTC] #2 phutil_tag(string, array, string) called at [<phutil>/src/markup/syntax/highlighter/PhutilLexerSyntaxHighlighter.php:55] [29-Jul-2014 01:54:03 UTC] #3 PhutilLexerSyntaxHighlighter::getHighlightFuture(string) called at [<phutil>/src/markup/syntax/highlighter/xhpast/PhutilXHPASTSyntaxHighlighterFuture.php:26] [29-Jul-2014 01:54:03 UTC] #4 PhutilXHPASTSyntaxHighlighterFuture::didReceiveResult(array) called at [<phutil>/src/future/FutureProxy.php:60] [29-Jul-2014 01:54:03 UTC] #5 FutureProxy::getResult() called at [<phutil>/src/future/FutureProxy.php:37] [29-Jul-2014 01:54:03 UTC] #6 FutureProxy::resolve() called at [<phutil>/src/markup/syntax/engine/PhutilSyntaxHighlighterEngine.php:11] [29-Jul-2014 01:54:03 UTC] #7 PhutilSyntaxHighlighterEngine::highlightSource(string, string) called at [<phutil>/src/markup/engine/remarkup/blockrule/PhutilRemarkupEngineRemarkupCodeBlockRule.php:185] [29-Jul-2014 01:54:03 UTC] #8 PhutilRemarkupEngineRemarkupCodeBlockRule::highlightSource(string, array) called at [<phutil>/src/markup/engine/remarkup/blockrule/PhutilRemarkupEngineRemarkupCodeBlockRule.php:132] [29-Jul-2014 01:54:03 UTC] #9 PhutilRemarkupEngineRemarkupCodeBlockRule::markupText(string, NULL) called at [<phutil>/src/markup/engine/PhutilRemarkupEngine.php:226] [29-Jul-2014 01:54:03 UTC] #10 PhutilRemarkupEngine::markupBlock(array) called at [<phutil>/src/markup/engine/PhutilRemarkupEngine.php:122] [29-Jul-2014 01:54:03 UTC] #11 PhutilRemarkupEngine::preprocessText(string) called at [<phabricator>/src/infrastructure/markup/PhabricatorMarkupEngine.php:254] [29-Jul-2014 01:54:03 UTC] #12 PhabricatorMarkupEngine::loadPreprocessorCaches(array, array) called at [<phabricator>/src/infrastructure/markup/PhabricatorMarkupEngine.php:122] [29-Jul-2014 01:54:03 UTC] #13 PhabricatorMarkupEngine::process() called at [<phabricator>/src/infrastructure/markup/PhabricatorMarkupEngine.php:67] [29-Jul-2014 01:54:03 UTC] #14 PhabricatorMarkupEngine::renderOneObject(PhabricatorMarkupOneOff, string, PhabricatorUser) called at [<phabricator>/src/infrastructure/markup/PhabricatorMarkupPreviewController.php:17] [29-Jul-2014 01:54:03 UTC] #15 PhabricatorMarkupPreviewController::processRequest() called at [<phabricator>/webroot/index.php:95]