Charles Ellicott Commentary Numbers 16:47

"And Aaron took as Moses spake, and ran into the midst of the assembly; and, behold, the plague was begun among the people: and he put on the incense, and made atonement for the people." — Numbers 16:47 (ASV)

And ran into the midst of the congregation. —The whole occasion was an extraordinary one. On ordinary occasions incense might only be offered on the golden altar within the holy place in which the priests ministered.