The Flash from Earth-3 is fighting the Trickster when Barry arrives to ask for advice on Savitar in episode 3x09 "The Present".
Source: What's Filming — Twitter
Neil Borman is signing up for a new account at the bank and the teller and patrons collapse as he is leaving in episode 4x10 "The Trial of the Flash".
The Fringe team arrives to see Gemini flying out the window in episode 3x06 "6955 kHz".
Etta meets with Nina in the lobby of the Ministry to talk about Walter in episode 4x19 "Letters of Transit".
Elizabeth Bishop talks to Nina about the dimensional bridge device that Walter is building in episode 2x16 "Peter".