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According to the parsing spec having "-->" in the region id metaheader is alright. This probably shouldn't be the case since it isn't valid for cue setting region ids.
Either this has been solved or I was wrong to being with because I'm going over the spec right now and don't see where '-->' is not allowed in cue-settings. It might make sense to make '-->' a reserved character sequence (possibly?), but other then that I think this bug can be closed.
OK, closing this bug.