What ends up happening is that the full URL is posted as plain text.
This usually isn't the end of the world as:
- at least the URL isn't auto-abbreviated (which happens in other cases), so it remains intact
- the poster can go back and edit the OP to linkify the URL manually
However, at some point the OP won't be editable, and any plain text URLs will remain that way forever.
I don't think this happens all the time, but it's happened more than once.
Example:
