Comment (by Roland Haas):

For the sake of documenting this: the value on cctk_nghostznones in LEVEL mode (Carpet only, the issue does not arise in PUGH) is well defined (ie not technically “garbage” as in “unitinialized memory” or “to be garbage collected”) and is 666 (by default) which is chosen to be a recognizable value that is likely to trigger errors when used.

See https://bitbucket.org/eschnett/carpet/src/master/Carpet/src/modes.cc#lines-528 and deadbeef is a CarpetLib parameter defaulting to 666.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2288/leans-boundary-condition-setting-for-bssn