I remember when "Trials and Tribble-ations" aired in late 1996. I want to say it aired a few weeks before First Contact was released. When Sisko is explaining the events, he mentions they were on the Enterprise. One of them says, "Be specific, Captain, which Enterprise? There have been five." and the other corrects him and says, "Six." -- suggesting the then-new Enterprise-E, which none of us had seen yet.