How often do fast food employees sabotage meals?

There are all these horror stories out there… The dominos incident, mcdonalds guys spitting on hamburgers, etc.

It’s totally gross and it wigs me out. It not like I’m a germaphobe; people’s bodily substances could really make someone sick!

Does sabotage really happen that often or does the media just blow it out of proportion? I guess this question is really for people who have worked in fast food, since they would know.