Here's a nice example of someone who doesn't understand what was going to happen. This tree tag was nailed flush to the trunk just a few years ago, and the tree grew. Some people think the growth of the tree via its vascular cambium will push the tag out, but no, the pressure is bending the tag, and eventually the trunk is starting to engulf the tag. This is generally not a good system for tagging a tree, although if an inch or so of slack were left the problem would not have developed so soon. Unfortunately when the tags are loose, people play with them, bending, and twisting them, etc. But now the injury might get bigger when someone attempts to pull the nail and reattach the tag. Even something as trivial looking as a thin wire or even tough cordage can apply enough pressure to cause the tree to grow out and around the object. Cables, chains, or fencing attached to a tree trunk can also be engulfed. Best practice is not to do it. Even the best trunk tags need to be reset every few years, and if you have lots of trees tagged, it becomes a big job.