There are a couple giveaways.
First, the head falls strait down, but is offset by at least 8" from where it should be. This looks like it was done so that it would be more visable and thus heighten the effect.
Second, just as the cat (headless) falls over and starts to slide down the windshield, the shadow doesn't make sense. One frame there is tons of shadow right above the tail, then almost no shadow, and no reflection of the cat in the window. Granted, in a frame or two, it does look like there is a hint of reflection, but it looks more like it was painted in. Another thing to note is when the cat starts to slide, there are 3 frame where the head (that C. mentioned) is crapped out of it's neck. Then it slides down the windshield as a whole cat. Also note the little antenna thing (nub) right in the center and just above the windshield. It looks pretty substantial, but the cat neither hangs nor is affected by it. To the cat, it just doesn't exist. Or rather since the cat doesn't really exist, it can't be affected by the hook like thing on the car (which is more than likely real).
Third, the bloody stump. Chris, there are 4 frames that show a headless cat. The, uh, frames show a plain red area encompassed by a thin white ring. I fail to see how such a wound can be so bloodless (also note there is no blood trail on the windshield) and so devoid of details. To me it looks digitally doctored.
The last thing that doesn't fit is how the cat slides off the car. It seems to me that it would take the most direct route to a lower potential energy state and head strait down the windshield then slower down the hood and land in front of the car. This IMO would have been way too gross leaving the dead cat in view. With it disappearing behind the car kind of sugar coats it. Maybe it would have been a tad too much to have the hood launch the cat towards the house at the end too. Just my opinion, but that would have been cool. Kind of like pinball.