Mesothelioma and other competitive SERPs.
I have an exact meaning, thank you. When I say "sandboxed" I mean a new domain with "unnatural" incoming links is seemingly penalized to the bottom of the SERPs (it is not grey bar's though). Older domains with these same "unnatural" links arent penalized.
Plenty of sites on the same IP do link to each other (with no harm), but I am stating that the SEO benefits of these links will be filtered. They should yield other benefits if they make sense for your users. And for new (< 6 months old) domains, I think this will greatly increase your chance of being "sandboxed".
Does this make sense according to the Hilltop paper? No. But Google is not behaving according to the Hilltop paper. IMO they are about 2 years past that, and have added a whole bunch of "SEO filters" into the algo.