Odd/unhelpful end-user error conditions & their possible remedy
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* User is able to choose installation type, but after
  clicking the "Install" button to kick off the copying
  of files etc, the dialog box "Cannot find the file
  specified" (or a variant thereof) with the buttons
  Retry/Cancel. Retry doesn't help, and on hitting Cancel
  the error code 2755, 110, Unable to open "<MSI filename>"
  
  The cause of this one was inappropriate permissions on the MSI
  itself. Enabling Read&Execute for everyone solves the problem.

* Error reporting for nested installs where the sub-installers
  aren't actually present (i.e., haven't been injected with 'msidb')
  isn't informative; the installer reports a fatal error after having
  completed the main installation.

* Starting up an .msi that's already opened elicits a dialog box
  informing the user that it couldn't be opened. The hints included
  in the dialog box text as to why that might be doesn't include the
  possibility that it may already be opened by another application.