Here is a scenario where the error handling of VirtualBox is broken:
1) add a shared folder to a virtual machine
2) remove the "real" directory while vbox is not running
3) restart VirtualBox
The user gets the following error message: "The snapshot folder of a machine with snapshots cannot be changed (please discard all snapshots first)."
First of all, the error message has absolutely nothing to do with the real problem (non-existance of a specified shared folder) and is very misleading.
Secondly, a missing folder is no fatal situation, it should be possible for a user to recover from this situation without manually editing some obscure XML files.