I thought this was pretty weird, and oddly specific. So of course, I looked it up.
DVT (Deep Vein Thrombosis) is a type of blood clot that occurs in deep vein of a lower extremity - such as the calf. Using the vibrator there could break the clot free from the wall and allow it to cause an embolism in heart, lungs, etc.
I don't remember ever seeing this warning before, though my search showed that it's on lots of them (and one source said the FDA says it HAS to be, but I'm not sure about the validity of that).
I'm not sure what's more alarming: that any of my vibrators could kill me if I had DVT and didn't know it.. or that I've been using sex toys for years now and have only just come across this information. Scary!