Amy says that she and Howard last spent time together three years ago. This is a reference to the seventh season episode The Scavenger Vortex (2013) that was aired 2.5 years before: it refers to when the two were paired together for the scavenger hunt created by Raj, or the Neil Diamond concert the two said they should go to.
Penny's ex-boyfriend Zach (Brian Thomas Smith) makes his first appearance since season 7's The Cooper Extraction (2013), and even seems a bit smarter than before.