Excepted that the actual behavior is the following: when a file (an image, as an example) is overwritten on this shared folder, Apache still serves the old content. I suspected some caching or timestamp issues, but I can only reproduce this behavior on a VirtualBox shared folder. The problem can also be reproduced my overwriting the file using a "cp" command while logged on the Debian guest.
If I perform the same manipulation of a "regular" guest directory, overwrites are taken into account as soon as the web page is reloaded, so this clearly seems related to shared folders. Interestingly, the only way I found to get the new content was either to reboot the VM, or to perform the following command to empty the OS cache:
sync; echo 3 > /proc/sys/vm/drop_caches
So my hypothesis is that this is a vboxsf linux module bug: when a file is overwritten, it correctly updates its disk content and its metadata (timestamp, size), but it fails to inform the OS that its cache for this file should be invalidated.
Did anyone meet something similar? Does my interpretation seem reasonable? I would prefer to discuss this issue here before logging a bug on the tracker.