When Jillian introduced her fellow Ghostbusters to new equipment, Erin was told to pick a piece of equipment. Erin had picked up the Proton Grenade Launcher, but Jillian told her it wasn't ready. Jillian then gave her the Swiss Army Knife and then said "No woman should walk around unarmed." and then after that had her test the Proton Grenade (Air filter version).
Later, Jillian, Abby, and Patty were pinned by the huge Stay Puft balloon. To save them, Erin used the Swiss Army Knife to pop it, saving the day.