The most common words to grace a cake are "Happy Birthday." Most people can spell "happy," but when it comes to "birthday," the danger of a brain fart increases. Jen Yates at Cake Wrecks has seen so many cases of bizarre spellings of that one word that she had to split a compilation into two posts, and the misspellings run the gamut of missing letters to completely different words to incoherent jumbles.
Having once worked in a supermarket, I know how this happens. Someone buys a blank cake and requests an inscription in icing. The bakery/deli department has as few people on duty as possible, and half of them are new, and the other half have avoided ever learning how to use icing. So a request goes out to the entire store for someone who's done it before, or is at least willing to try. Bob in the loading dock sees a chance to spend a few minutes in the air conditioning with no heavy lifting, and volunteers. And if any volunteer sees a misspelling, it is way easier to pretend not to see it than to fix it. The customer? They have a ready-made submission to Cake Wrecks.
See how people can can misspell "birthday" in ten ways here and eight more ways here.