Qt relies on FrameLoader::checkLoadCompleteForThisFrame() to load error pages. backtrace: (...) a load error happens (...) 1) FrameLoader::checkLoadCompleteForThisFrame() 2) FrameLoaderClientQt::dispatchDidFailProvisionalLoad 3) FrameLoaderClientQt::callErrorPageExtension 4) an "error page" is set via FrameLoader::load(request, substituteData, lockHistory) note that we're yet in the middle of _1_ , however another load has happened (the error page has been loaded in the same loader object). so the previous m_provisionalDocumentLoader has been changed. In details, see quoted code below: void FrameLoader::checkLoadCompleteForThisFrame() { RefPtr<DocumentLoader> pdl = m_provisionalDocumentLoader; if (!pdl) return; // If we've received any errors we may be stuck in the provisional state and actually complete. const ResourceError& error = pdl->mainDocumentError(); if (error.isNull()) return; bool shouldReset = true; <--- THIS IS IMPORTANT if (!(pdl->isLoadingInAPISense() && !pdl->isStopping())) { m_delegateIsHandlingProvisionalLoadError = true; m_client->dispatchDidFailProvisionalLoad(error); m_delegateIsHandlingProvisionalLoadError = false; (...) // READ THE COMMENT BELOW !!!! // Finish resetting the load state, but only if another load hasn't been started by the // delegate callback. if (pdl == m_provisionalDocumentLoader) clearProvisionalLoad(); else if (m_provisionalDocumentLoader) { KURL unreachableURL = m_provisionalDocumentLoader->unreachableURL(); if (!unreachableURL.isEmpty() && unreachableURL == pdl->request().url()) shouldReset = false; } (...) in our qt specific case "another load" *has* happenned *but* the "else" statement just checks for 'm_provisionalDocumentLoader'. That breaks our back / forward action because it does get reset. My solution: we could check for the current active document loader, not only for the m_provisionalDocumentLoader, so using activeDocumentLoader() ... patch coming