Almost everytime Gudelhian and Heinel fight. Especially in episode 8 of the TV series, when after an argument, they enage into a physical fight.
It gets sillier. In the first episode of SIN, they get into a fight while trying to endorse a racing game, ostensibly a Cyber Formula simulator. A caption pops on as they go at each other over how no, they're not selling a fighting game.
When Shinjyo and Miki returned to Cyber Formula as members of Union Savior in EP 6 of SAGA by wearing disguises, Hayato and Asuka instantly recognized them. Shinjyo and Miki's reactions were priceless.