In the past decade Grid computing has matured considerably. A number of groups have built, operated, and expanded large testbed and production Grids. These Grids have inevitably been designed to meet the needs of a limited set of initial stakeholders, resulting in varying and sometimes adhoc specifications. As the use of e-Science becomes more common, this inconsistency is increasingly problematic for the growing set of applications requiring more resources than a single Grid can offer, as spanning these Grid islands is far from trivial. Thus, Grid interoperability is attracting much interest as researchers try to build bridges between separate Grids. Recently we ran a case study that tested interoperation between several Grids, during which we recorded and classified the issues that arose. In this paper we provide empirical evidence supporting existing interoperability efforts, and identify current and potential barriers to Grid interoperability.