Occasionally (in an indeterminate manner) an application using WebKit will crash with the following stack trace: #0 0x021ea2cd in WTF::RefCountedBase::derefBase () #1 0x0235071d in WTF::RefCounted<WebCore::StringImpl>::deref () #2 0x0235075f in WTF::derefIfNotNull<WebCore::StringImpl> () #3 0x023fc4e1 in WTF::RefPtr<WebCore::StringImpl>::~RefPtr () #4 0x0235079d in WebCore::String::~String () #5 0x02413113 in WebCore::MediaQueryEvaluator::~MediaQueryEvaluator () #6 0x023cec15 in WebCore::CSSStyleSelector::~CSSStyleSelector () #7 0x0242c9ab in WebCore::Document::~Document () #8 0x02429818 in WebCore::Document::removedLastRef () #9 0x023797c5 in WebCore::TreeShared<WebCore::Node>::deref () #10 0x022f159a in WTF::derefIfNotNull<WebCore::Document> () #11 0x024240f3 in WTF::RefPtr<WebCore::Document>::~RefPtr () #12 0x025e2fed in WebCore::Loader::Host::didFail () #13 0x025e31d5 in WebCore::Loader::Host::cancelRequests () #14 0x025e33a9 in WebCore::Loader::cancelRequests () #15 0x025be2d6 in WebCore::DocLoader::~DocLoader () #16 0x0242bdc8 in WebCore::Document::~Document () #17 0x02528121 in WebCore::HTMLDocument::~HTMLDocument () #18 0x0243567e in WebCore::Document::selfOnlyDeref () #19 0x024297fe in WebCore::Document::removedLastRef () #20 0x023797c5 in WebCore::TreeShared<WebCore::Node>::deref () #21 0x022f159a in WTF::derefIfNotNull<WebCore::Document> () #22 0x0244b651 in WTF::RefPtr<WebCore::Document>::operator= () #23 0x02632d4c in WebCore::Frame::setDocument () #24 0x025d050c in WebCore::FrameLoader::clear () #25 0x025d34c9 in WebCore::FrameLoader::begin () This appears to occur when navigating away from a page that contains CSS (even if there is no explicit media query). In this case, the application (Google Earth) is using QtWebKit (4.6.3) and navigating to "about:blank", but I do not think this is a Qt-specific bug.