Currently, when a forum post is deleted (either by the user themselves, or by a moderator), the post number will be reused for a new post. This means that a URL that once led to one post, now suddenly leads to another.
As an example, the below post was a spam post about sweaters. You can see the question ID is 109377:
However, if you open the URL to this question ID now, it leads to an entirely different post:
This can lead to confusing situations where you're linking something you didn't mean to link to at all. An easy solution would be to reserve the IDs of deleted post so that they cannot be reused.