If I use QFileDialog for opening files, it shows thumbnail previews of the files (tested with Qt 4.7.4 on Gnome on Ubuntu Linux 11.10). However, it does not check if the thumbnails really belong to the current version of the file. According to the thumbnail standard from freedesktop.org, it needs to compare the Thumb::MTime property stored in the PNG of thumbnail with the last modification time of the file the thumbnail belongs to.

This bug can be reproduced by using any Qt application that uses a QFileDialog for opening files, looking at a file that has a thumbnail, then changing the file without that the thumbnail gets updated by the desktop environment (Gnome), for example by overwriting it with a different file using cp in a terminal and looking at the thumbnails in the QFileDialog again.