I saw this today.

Problem:
A comma is missing.

Explanation:
Expressions of happy-birthday wishes are usually abbreviations of a longer sentence along the lines of “Have a happy birthday, Kirk!”, with a comma immediately after the word “birthday” to indicate the pause in speech.

Even though the imperative sentence is abbreviated to drop the “Have a” part of the sentence, the comma should persist.

And, even if one does not appreciate the abbreviation, it should still be clear that a comma is required because of the pause between “Happy birthday” and the person’s name.

Solution:
“Happy birthday, Kirk!”