When you call the .toString() method, sometimes it returns an incorrect string.
Example :
--> 1 degree - 8 minutes - 0 seconds --> as double is 1,13333333333
--> you create a QGeoCoordinate X( 1,13333333333, 0 )
--> .toString() method returns [ 1º 7' 60" ] ... ( 7' + 60" == 8' ) ...

The strange thing is that this happens with certain values.
I'fixed this by subclassing QGeoCoordinate and implementing my own toString() method.