The incident took place during the band's show at the Annexet in Stockholm, Sweden on July 31.
Posted on Reddit, the clip shows the frontman initially appearing to take the drunken comments in his stride, and sees him flip a light-hearted middle finger in the fan's direction. However, as the gentleman in question continues to shout out irritating remarks, Mustaine gets increasingly worked up and starts to repeatedly tell him to "shut up".
At one point, Megadeth's leader is visibly pissed off when the fan suggests that he is capable of playing guitar as well as him.
After Mustaine threatens to call security to remove the man from the venue, the heckler finally stops bothering him, and Mustaine tells the audience: "See, that’s what happens when cousins fuck." Well, that's them told.
Comments underneath the video add more context to the spat, with one user writing that the drunk fan was being "loud and obnoxious by interrupting Dave during one of his speeches, saying they wanted to go up and play with him."
Another says: "I was at this show and they were purposefully knocking people over. After about like 40 minutes I just smacked one of them, because they'd knocked my dad to the ground and kept rubbing my back".
The moral of the story here is that drinking at gigs is fine, as long as you're not disturbing those around you. And never, ever suggest that you can play guitar as well as Dave Mustaine.
Megadeth's next performance will see them play Wacken Open Air in Germany today, August 4.
Check out the footage of the incident below: