Created attachment 72470 [details]
sample code of the test case

Created attachment 72471 [details]
crash backtrace of the test case

Also you can use this use case: Is the same of the kdelibs/kdeui/test but this time we add more items after the view has been setup, please also note the comments about setCategorizedModel; so here is *another use case*:

-- CUT FROM HERE ---


    #include <QMainWindow>
    #include <QStringListModel>
     
    #include <kapplication.h>
    #include <kaboutdata.h>
    #include <kcmdlineargs.h>
    #include <kiconloader.h>
     
    #include <kcategorizedview.h>
    #include <kcategorydrawer.h>
    #include <kcategorizedsortfilterproxymodel.h>
     
    QStringList icons;
     
    class MyModel
        : public QStringListModel
    {
    public:
        virtual QVariant data(const QModelIndex &index, int role) const
        {
            switch (role) {
                case KCategorizedSortFilterProxyModel::CategoryDisplayRole: {
                        return QString::number(index.row() / 10);
                    }
                case KCategorizedSortFilterProxyModel::CategorySortRole: {
                        return index.row() / 10;
                    }
                case Qt::DecorationRole:
                    return DesktopIcon(icons[index.row() % 4], KIconLoader::Desktop);
                default:
                    break;
            }
            return QStringListModel::data(index, role);
        }
    };
     
    int main(int argc, char **argv)
    {
        icons << "konqueror";
        icons << "okular";
        icons << "plasma";
        icons << "system-file-manager";
     
        KAboutData aboutData("KCategorizedViewTest",
                             0,
                             ki18n("KCategorizedViewTest"),
                             "1.0",
                             ki18n("KCategorizedViewTest"),
                             KAboutData::License_LGPL_V3,
                             ki18n("(c) 2009 Rafael Fernández López"),
                             ki18n("KCategorizedViewTest"),
                             "http://www.kde.org");
     
        KCmdLineArgs::init(argc, argv, &aboutData);
        KApplication app;
     
        QMainWindow *mainWindow = new QMainWindow();
        mainWindow->setMinimumSize(640, 480);
        KCategorizedView *listView = new KCategorizedView();
        listView->setCategoryDrawer(new KCategoryDrawer());
        listView->setViewMode(QListView::IconMode);
        MyModel *model = new MyModel();
     
        model->insertRows(0, 100);
        for (int i = 0; i < 100; ++i)
        {
            model->setData(model->index(i, 0), QString::number(i), Qt::DisplayRole);
        }
     
        KCategorizedSortFilterProxyModel *proxyModel = new KCategorizedSortFilterProxyModel();
        proxyModel->setCategorizedModel(true);
        proxyModel->setSourceModel(model);
     
        listView->setModel(proxyModel);
     
        //NOTE if you disable temporally the CategorizedModel flag work ok but the scroll bar fails
        //proxyModel->setCategorizedModel(false);
     
        //BEGIN HERE
        model->insertRows(0, 100);
        for (int i = 0; i < 100; ++i)
        {
            QModelIndex mi = model->index(0,0);
            model->setData(model->index(i, 0, mi), QString::number(i), Qt::DisplayRole);
        }
        //END HERE
       
        //NOTE if you reenabled the flag works fien but the scroll bar fails
        //proxyModel->setCategorizedModel(true);
     
       
        mainWindow->setCentralWidget(listView);
     
        mainWindow->show();
     
        return app.exec();
    }

--- UNTIL HERE ---

Gave it a short test:
case KCategorizedSortFilterProxyModel::CategorySortRole seems never be queried and the proxymodel most likely runs into a race on the items (ie. with i<2 everything is fine since there're only two non equal items and so is if you pass QString::number(index.role()) as display role.

Don't ask me why, i got the nature of the bug wrong and then looked out of curiosity. I've never touched or seen the kcat'model code. Sorry.