On windows, the following code causes an assert in QFileSystemModelPrivate::addNode. This is just an easy way to reproduce a problem that can have severe consequences.
For instance, I use a QSortFilterProxyModel::filterAcceptsRow and in certain cases, my overriden function is called recursively, causing sometimes a crash.

#include <QApplication>
#include <qfilesystemmodel>
int main(int argc, char* argv[])
{
QApplication app(argc, argv);
QFileSystemModel model;
model.setRootPath(QLatin1String("\\\\MACPRO\\Data"));
model.setRootPath(QLatin1String("\\\\MACPRO\\Data"));
}

The root of the problem is in QFileSystemModelPrivate::node (and maybe elsewhere) where the case sensitivity of the file system is not taken in account,

There is this line:

 if (!root.children.contains(host.toLower())) { 

while in other parts of the file, the case sensitivity is taken into account, eg.

if ((parent->children.count() == 0)
    || (parent->caseSensitive()
        && parent->children.value(element)->fileName != element)
    || (!parent->caseSensitive()
        && parent->children.value(element)->fileName.toLower() != element.toLower()))

[EDIT] there are other reports that seem to be related. But here my little sample is very easy to test.
QTBUG-21278, QTBUG-31103, QTBUG-61786, QTBUG-17750