Beam Me Up, Scotty!: Dethklok is not the sixth/seventh largest economy in the world, just above Belgium. In the first season, "[Dethklok is] now the twelfth largest economy in the world, having just surpassed Belgium", in the second season they are declared the seventh largest economy in the world, in the third season, "[Dethklok is] now the sixth largest economy in the world". It can be assumed that they have become the World's Top Economy by Season Four, since they send the entire planet's economy spiralling downwards after all the copies of their new album end up lost/destroyed.
Defictionalization: Brendon Small has done several live tours as Dethklok, not to mention released The Dethalbum, which has the honor of being the highest-charting Death Metal album in Billboard's history. There have since been two more Dethalbums and a fourth for The Doomstar Requiem.
The Dethklok Fountain has now also been defictionalized, for those of you with $40,000 to drop on Adult Swim merchandise.
Executive Meddling: Season 3 was shockingly tame compared to the first season; this was due to the fact that Adult Swim ordered the carnage be toned down, since Superjail (which had a similarly bloody tone) had debuted and they wanted to avoid people comparing it to Metalocalypse as far as ripping off it's tone.
Fan Nickname: After Charles returns at the end of the third season premiere with his newly acquired hero scar, several fans dubbed this even more badass version of him "Scarles".
Each of the members of SnB is voiced by a musician: Thunderbottom by Michael Amott (Arch Enemy guitarist), Candynose by Warrel Dane (Nevermore vocalist), and Snazz by Steve Smyth (Forbidden guitarist). Ironically enough, only Snazz's instrument of choice matches his voice actor's.
The Other Darrin: Tommy Blacha was unavailable for the Doomstar Requiem special, so Toki's singing was provided by Mike Keneally, with Brendon Small filling in for younger Toki. Murderface and Dr. Rockso also had their singing provided by Brendon Small.
Talking to Himself: Brendon Small, Tommy Blacha, Mark Hamill, Up to Eleven. Go ahead, check out Wikipedia. See how many characters these people voice, not including background characters. Regular voices are more than double the regular voice actors. The entire band and CFO are voiced by two people.