Sunday SciFi: Ringoverse/Warehouse 13 crossover
OK, it’s more of a ‘guest appearance’ than a reference. Artie and Claudia show up to bag and tag an artifact after Barbara Everette, and an Opus Dei strike team, finish wiping out the nest of evil people who wanted to use it to raise a demon. If you have read the first Special Circumstances book, the only spoiler there concerned Artie and Claudia.
Apparently, John Ringo is a fan of the show.