A tongue barbell that is too short could cause the jewelry to embed itself in the tongue. This is especially true in new tongue piercings. Jewelry that is too short also has a tendency to cause pain and swelling.
The easiest way to tell if a tongue barbell is too short is to look in the mirror. If more than half of the top ball of the jewelry is nesting into the tongue when the tongue is completely at rest, the barbell is too short and at risk for possibly embedding. In new piercings, extra-long barbells are often needed to accommodate swelling.