If you Ctrl-C during a vbox build, packer is unable to clean up. Until it is manually cleaned up, it is also unable to create that build target:

==> centos-6.5-x86_64: Unregistering and deleting virtual machine...
==> centos-6.5-x86_64: Error deleting virtual machine: VBoxManage error: VBoxManage: error: Cannot unregister the machine 'centos-6.5-x86_64' while it is locked
==> centos-6.5-x86_64: VBoxManage: error: Details: code VBOX_E_INVALID_OBJECT_STATE (0x80bb0007), component Machine, interface IMachine, callee nsISupports
==> centos-6.5-x86_64: VBoxManage: error: Context: "Unregister(CleanupMode_DetachAllReturnHardDisksOnly, ComSafeArrayAsOutParam(aMedia))" at line 158 of file VBoxManageMisc.cpp
==> centos-6.5-x86_64: Deleting output directory...
Build 'centos-6.5-x86_64' errored: Build was cancelled.
Cleanly cancelled builds after being interrupted.

jkyle at metaverse in ~/Projects/ISOs/galra (feature/virtualbox●●●)
(packer)$ packer build -only=centos-6.5-x86_64 -var 'proxy=http://172.16.101.186:8000/' templates/redhat_virtualbox.json
centos-6.5-x86_64 output will be in this color.

==> centos-6.5-x86_64: Downloading or copying Guest additions checksums
centos-6.5-x86_64: Downloading or copying: http://download.virtualbox.org/virtualbox/4.3.6/SHA256SUMS
==> centos-6.5-x86_64: Downloading or copying Guest additions
centos-6.5-x86_64: Downloading or copying: http://download.virtualbox.org/virtualbox/4.3.6/VBoxGuestAdditions_4.3.6.iso
==> centos-6.5-x86_64: Downloading or copying ISO
centos-6.5-x86_64: Downloading or copying: http://mirror.stanford.edu/yum/pub/centos/6.5/isos/x86_64/CentOS-6.5-x86_64-minimal.iso
==> centos-6.5-x86_64: Creating virtual machine...
==> centos-6.5-x86_64: Error creating VM: VBoxManage error: VBoxManage: error: Machine settings file '/Users/jkyle/VirtualBox VMs/centos-6.5-x86_64/centos-6.5-x86_64.vbox' already exists
==> centos-6.5-x86_64: VBoxManage: error: Details: code VBOX_E_FILE_ERROR (0x80bb0004), component Machine, interface IMachine, callee nsISupports
==> centos-6.5-x86_64: VBoxManage: error: Context: "CreateMachine(bstrSettingsFile.raw(), bstrName.raw(), ComSafeArrayAsInParam(groups), bstrOsTypeId.raw(), createFlags.raw(), machine.asOutParam())" at line 275 of file VBoxManageMisc.cpp
==> centos-6.5-x86_64: Deleting output directory...
Build 'centos-6.5-x86_64' errored: Error creating VM: VBoxManage error: VBoxManage: error: Machine settings file '/Users/jkyle/VirtualBox VMs/centos-6.5-x86_64/centos-6.5-x86_64.vbox' already exists
VBoxManage: error: Details: code VBOX_E_FILE_ERROR (0x80bb0004), component Machine, interface IMachine, callee nsISupports
VBoxManage: error: Context: "CreateMachine(bstrSettingsFile.raw(), bstrName.raw(), ComSafeArrayAsInParam(groups), bstrOsTypeId.raw(), createFlags.raw(), machine.asOutParam())" at line 275 of file VBoxManageMisc.cpp