During the original design process, an unnamed scientist who worked with Robotnik on the formula for Mega Mack was exposed to large quantities of the chemical. Rather than being killed, this individual somehow mutated and merged with the substance, becoming the maniacal creature known as Megatox. Megatox was completely one with the Mega Mack, his consciousness capable of manipulating any amount of the chemical added to his original body. He was capable of liquid-shifting, splitting into several bodies and later could make himself solid by absorbing large amounts of electrical energy.
Nutzan Bolt's Chemical Plant Plot
Under orders from Doctor Robotnik, Nutzan Bolt attempted to demolish the Chemical Plant Zone's primary chemical silo, releasing a torrent of Mega Mack into surrounding zones, killing everything. His scheme was thwarted by Tails and The Flock, who destroyed Nutzan's demolition robots and used the molten slag from their bodies to seal up the breach in the silo.
Sometime after Robotnik's downfall, the doctor's former lackeys - the Marxio Brothers - took control of the Chemical Plant Zone. Despite Sonic's suspicions, Grouchio and his brothers claimed that their business was legitimate and that they intended to clean up the Mega Mack from the zone. Despite this promise, Mega Mack has still been produced and used by different parties. In Sonic the Comic Online, Grimer was attempting to destroy Sonic's reputation and prior to his capture of Sonic had deployed a Mega Mack bomb in the Green Hill Zone. KBC's broadcast of the event made out that Sonic had deployed the bomb and was escorting it to its target.