Light command randomly fails (Wix 2.0)

We've recently started to see errors on our production build machines
of the form:

Failed to create cab 'C:....\Local\Temp\-zymqxqr\MergeModule.CABinet'
with error 0x80070050.

On looking in the temp folder it was full of old files seemingly left
behind after earlier builds. The solution appears to be to manually
clean up this folder...