In fairness, you've only got yourself to blame for this one, as right near the start of the game, you're given a situation that can only end with a cow being launched into space. Brilliantly, though, you've probably more or less forgotten about it by the end of the game, where said bovine missile crashes back down to terra firma, squashing the lovely lady you've been trying to reach and preventing the obligatory kiss for the hero from happening.
What would really happen? The cow would probably arrive burnt to a crisp having already burned up travelling through the atmosphere (unless he's related to Master Chief... which would explain why we never see Chief's face), meaning everyone could enjoy a nice BBQ beef burger. Either that or it would explode on impact. Either way, nothing funny about it. Just a lot of mess to clean up.