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So I accidentally installed an ARM base system in a AMD64 jail and tried 
to start it. It doesn't work, no one will be surprised to know.

The thing is I'd expect service to return non-zero if the jail it's 
starting crashes and burns. To quote the FM:

" The service utility exits 0 on success, and >0 if an error occurs."

A pretty major error, but it still returns success. Now I can see why 
this might be considered success - the "service" utility did what it was 
told and the service itself went badly wrong. But it's not exactly 
helpful if you want to know if your jail has started properly, is it?

My question - is this a bug, or can someone explain why it's actually a 
feature?

Thanks, Frank.