http://tools.wmflabs.org/ has "add / remove maintainers" links to Special:NovaServiceGroup that don't supply an returnto parameter. After successful addition of another user to the service group, Special:NovaServiceGroup outputs the back link as "<!-- ERROR -->Back to project list".
I'm not sure whether we would want to allow arbitrary URLs for returnto, or just provide a sensible default (Nova_Resource:$PROJECT?).
Version: unspecified
Severity: trivial