In the ice-hockey episode, (where the guy gets an ice skate to the neck) Gil goes to examine the wound in the dead guys neck and you can see the dead guy's throat move as he swallows!
In CSI they have to use live people for all of the dead bodies so that they can do the scenes for the flashbacks when they are alive. There's bound to be a little slip-up like this with using live people.
If you watch any type of show thats forsenics or something like Law and Order, they don't use actual dead bodies. You can definately see that when they use young children.
Using live people isn't a good excuse. They can slow the frame, the person can lie flat and hold their breath (and not swallow) for the few seconds they show them. If a dead body swallows or breaths, it's a slip up. Period.