VSS Get & Checked out files?
 

I 've been tricked by this too many times :-(
Is it possible to fail a VSS Get step, when one or more files are checked out? (And yes, I've turned off 'ignore step failure)

If not, how can one detect checked out files in advance?

A VSS status call on the files in question will not fail if no files are checked out, and will fail if any files are checked out. So you should be able to add a Source Safe status step before the Checkout step to get that behaviour. See the SourceSafe.bld sample for an example Status step.