Love it or hate it, the most popular line you'll hear on Overwatch when Mei is around is the dreaded sound of her ice wall accompanied by a sometimes sincere, most times ill-intentioned string of sorries. 

Given the nature and repercussions of Mei's abilities it makes absolute sense to have that voice line in there. However, Overwatch Senior Game Designer Michael Chu, shared during the Tribeca Games Festival that the line, which is also his favorite in the game, was actually an outtake from a fortuitous recording booth error that happened while working with Mei's voice actress, Zhang Yu.

He told the audience:

And the rest is Overwatch history. I always enjoy hearing these stories about the game development process where even the most random, seemingly insignificant things end up playing a long-term role in a game like setting the tone for what the image and persona of a character will become. Even if in this case it was to provide the perfect line for trolling with a seemingly sweet and innocent character.

Source: Polygon