Ever assign a global definition to your worksheet, and then reference the worksheet elsewhere in another worksheet having the same global so it could otherwise stand on its own? If both have the same global assignment, then the Inclusion never resolves, and you spend a long while trying find a mysterious error that doesn't exist on its own. For example, I usually make standard unit definitions as a global (e.g. microJoules, uJ, picoJoules, pJ, etc... interesting how these are included standards a seconds, volts, amps, or watts, but not joules... nor femtoAmperes, fA for that matter) although sometimes I will assign global "DEFINE"s to control functional behavior.
So, rather than throwing an error and not calculating ANYTHING, a repeated global assignment should be treated as a warning only and have a default behavior to replace universally or just be ignored.
