Ok, full disclosure, this story is one that I have known about for a good few years, I remembered it when I was researching The Origins of Hamburgers. However, I thought it was an Urban Myth (it isn't) and that it was about McDonald's the UK (it isn't).
Rumours began circulating that McDonald's used worm meat in their burgers. Although no one can really be sure where these started from the most likely answer is that a fishing bait company declared they wanted to become the McDonald's of their industry and someone misread the quote and managed to put McDonald's and worms together.
At first, they decided to try and deal with these malicious stories locally but eventually, it affected the profits and they released a statement denying they put anything extra into the burgers - interestingly they refer to "Night-crawlers" in the statement, as apparently, the Ray Kroc felt that any mention of worms from an official McDonald's source would only add fuel to the rumours.