Wrong thumbnails are displayed for my photos in folder X, which happen to share the same name (the camera's counter rolled over) with older photos in folder Y. The thumbnails for the new photos show the old photos instead. Thunar 1.6.3, XFCE, OpenSuse 13.2
Do you have a reproducible test case? I've seen this, but only rarely (I think while doing some moves or editing or something), and haven't been able to get it to happen each time.
Please update to thunar 1.6.15 and check if it is fixed there ( Bug# 12435 ) @unhammer The bug you describe is exactly Bug# 12435 ... however the one from bogdan could be something else .. did not see it happen across different folders so far.
I cannot reproduce the "same thumbnail for same name" bug using thunar 1.8.x. So I assume it got fixed meanwhile ( either in thunar or in thumbler ). If this should not be the case, please feel free to reopen.