It was a big scare for this lady I used to work with too.
This was several years ago and some people were eating a box of M&M's in the lobby. They went to their rooms and left the box which still had some in it. So, as a joke, I asked, "Hey they left this box of M&M's and there are some left do you want some?" and she said, "Oh no, they could have razor blades in them"
And it was kind of WTF for several reasons. I saw them there sitting them eating them, who has razor blades small enough to put in an M&M? and so on. Plus it was just a joke!