Look on my works, ye Mighty, and despair!

If you take a copy of a VirtualBox hard disk (.vdi file), in the hope of attaching it as another VM, you may receive an error similar to this:

Failed to open the hard disk /home/blah/blah/blah.vdi.

Cannot register the hard disk 'home/blah/blah/blah.vdi' {some-guid} because a 
hard disk 'home/blah/blah/foo.vdi' with UUID {some-other-guid} already exists.

Result Code: NS_ERROR_INVALID_ARG (0x80070057)
Component: VirtualBox
Interface: IVirtualBox {c28be65f-1a8f-43b4-81f1-eb60cb516e66}

Thankfully the internets soon provided an answer, although the syntax has changed slightly:

VBoxManage internalcommands sethduuid blah.vdi

(Although the correct solution, of course, is to use VBoxManage clonehd in the first place!)