URL length limit enforced on edit but not on initial post

I tried to edit the content "Shout:" of a shoutbox post, but after clicking the Update button, the following message was displayed:
"URL cannot be longer than 55 characters but is currently 73 characters long."

I'm not even sure why there is a 55 character limited imposed here, but assuming that is an intended limit, shouldn't the length be checked when the shout is initially posted as well?