I was going to say this is a validator error on the clientside, but what should be mentioned is that you can see this pass clientside validation, because the client gets as far as attempting to submit post content:
There’s numerous ways to bypass the limit. Bottom line, it’s not important or worthwhile to fix. Users abusing this will be moderated appropriately. The limit is to make you think harder about what you’re posting, or if you should just use the like button.