One specific moment occurred during the recent "Big Four" tour with fellow thrash pioneers Megadeth, Anthrax, and Slayer, with all four bands performing a portion of Diamond Head's classic "Am I Evil?" on the same stage, in unison. To see these four bands actually performing in the same show together had to be something special, and a crowning moment for all involved.
Actually, it was only Metallica, Megadeth, and Anthrax playing "Am I Evil?" because the guys from Slayer were backstage editing their concert footage for the satellite broadcast.