This rarely happens (in fact, in the year I've been using Munki so far, this has happened only once), but you may once in a blue moon get a corrupted download (especially for a large installer). Here is an example of what that looks like:
ERROR: Error: "hdiutil: attach failed - image data corrupted" while mounting ExtremelyLargePackage-1.2.dmg.
ERROR: No mountable filesystems on ExtremelyLargePackage-1.2.dmg
So go to /Library/Managed Installs/Cache, where you should find the corrupt download. Re-do your managedsoftwareupdate check and a new, uncorrupted download should start.
Note: as far as I can tell, Munki will clear out the cache once downloads are successfully installed, so you shouldn't have to manually clear out the cache unless you run into this unlikely situation.