Let's have a simple example:

#include <QtCore>
#include <QtGui>

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    const QString html = "%1";
    QLabel l;
    l.show();
    QToolTip::showText(QPoint(120,200), html.arg("C:\\path\\to\\some\\c-file.cpp"));
    app.exec();
}

This shows a QLabel and a QToolTip with the path to a file. Looks nice! (plain.png)
But wait, what if the path is longer? Line breaks in a file path would look really ugly. To be sure, let's insert HTML's <nobr> attribute:

#include <QtCore>
#include <QtGui>

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    const QString html = "<nobr>%1</nobr>";
    QLabel l;
    l.show();
    QToolTip::showText(QPoint(120,200), html.arg("C:\\path\\to\\some\\c-file.cpp"));
    app.exec();
}

But what's this? Now there ARE two line breaks even though the path didn't change at all! (nobr.png)
Inserting <nobr> leads to line breaks which is the exact opposite of what it should do.

This obviously has something to do with the colon ( : ) and the dash ( - ). If we replace them, line breaks are gone. Even if they are replaced with white characters!