Let me explain. In the show, Shawn and company were tracking down evidence for a murder that was committed five years earlier to prove that the guy convicted for the killing is innocent. Shawn realizes that the scene, as it was told by the cops, couldn't be right - the dead guy had to have been shot from a different angle, meaning that the bullet would have flown out the window, explaining why the investigators never found it. They then set out to find the missing bullet which is likely lodged in one of the many trees just outside. Despite their best efforts, they can't seem to find the bullet hole, until Shawn's best friend reminds him that Shawn is looking at the wrong height - it's been five years since the shooting, after all. Low and behold, when they look a little higher, they find the bullet, but are chased off by gunfire before they can pull it out to find out who the real killer was.
Do you see it? Do you know what had me yelling at the screen?
No, it's not the odds that the bullet would be visible from 30 or so feet below five years later (though, really?). It's that the bullet somehow magically moved up a yard or two in five years. In fact, I'd be upset if it moved up in a hundred years. Let me explain.
Though it might seem intuitive that something placed at a certain height on a tree would move upward as the tree grows, that's simply not how trees do it. If it did, that would mean the tree is growing from the bottom, pushing the older tissues upward over time. Trees don't grow from the bottom, they grow from the top and outwards. What do I mean by that?
As a tree ages, it adds tissue to a limited number of places called meristems. These specialized areas of tissue occur at the tips of roots and branches as well as in the trunk of the tree, and are full of what you might call plant stem cells. They can differentiate to become any kind of plant tissue, which is exactly what you'd need to have wherever a plant needs to grow. The top of the tree, where a special meristem called the apical meristem occurs, is where height is added by these special cells dividing and differentiating.
While there are meristems in the trunk of the tree, they don't expand vertically, they expand horizontally. Once trunk tissue is laid down, it doesn't move upwards, only outwards, as new layers of secondary xylem and phloem (the vascular plant tissues that make up wood) are added. This is where tree growth rings come from - the yearly addition of layers of tissue to the trunk of the tree. It's why lovers can carve their names into a tree and come back and find the scars they created 50 years later in the exact same place, though the tree might be a little wider and taller than they recalled. It also means that no amount of tree growth would make a bullet lodged at a given height in the trunk move upwards.
What would actually happen to an object stuck in a tree over time? Something like this:
The tree trunk can actually grow around an object, so if anything, Shawn's missing bullet would have been deeper embedded and harder to see, not 5 feet higher up.
Am I strange to be so upset by this? Eh, maybe. But it bothered me that they could get something so small and simple wrong. I know, I know, it's just a TV show, but it's a pretty big biological mistake on the writers' part. If trees everywhere could yell at them, I'm sure they would.