Hey John,
Let's say the version installed on that computer is version 1.0 of your app. Let's also say the update is version 1.1 and that file has been changed. The reason you're getting the "Patch failed to apply [...]" error is that the file "dbupdate_5.0.sql" you included in version "1.0" in wyBuild is different that what is installed on the user's machine.
Does this make sense?