McDonald's' new 'Big Arch' burger is at the centre of the debacle (Image credit: McDonald's) When McDonald's CEO Chris Kempczinski sat down to promote the brand's new 'Big Arch' burger earlier last year, both he and the marketing team probably expected a nice, routine, light-hearted bit of forgettable social media content. Instead, they got a case study in how quickly brand messaging can spiral online. Posted in early February (although it didn't go viral until March), the clip shows Kempczinski taking a notably small, nay, minuscule, bite of the massive burger before delivering a carefully scripted endorsement. The whole thing didn't exactly scream "authentic", and ended up receiving the mother of all roastings online. From Kempczinski's hesitation to his repeated use of the word "product", the video was an embarrassment of riches for online trolls, and, of course, fast food rivals. Burger King was quick to respond with a tongue-in-cheek video featuring its own CEO confidently biting into a Whopper.…