As a side note: this issue happened to some early Apple watches (basically the glue didn't cure properly, I think?). Since it's a sorta known issue, you might be able to get it fixed for free, even though it's out of warranty.

Here's a tweet from John Gruber of popular Apple blog Daring Fireball, who had the same thing happen to his wife's watch: https://twitter.com/gruber/status/783016045481033729?lang=en