If you're using golang/dep, and for an unexplained reason running dep ensure
errors:
unable to update checked out version: : command failed: [git checkout 019ae5ada31de202164b118aee88ee2d14075c31]: exit status 1
You have to remove the dep from your Gopkg.lock file. This way Gopkg won't look for the missing ref.
And then dep ensure
again.
If that still doesn't work, and you're running 0.3.2 or earlier then try removing your cache:
$ rm -rf $GOPATH/pkg/dep/sources/
And then dep ensure
again.
--
The issue is likely that the/a project you depend on has forced pushed over the ref dep has cached (which it stores in $GOPATH/pkg/dep/sources/*).
The Kubernetes project is particularly fond of force pushing.