A relatively new complication of aligner therapy is emerging in the literature. Many clinicians have witnessed unplanned tooth movements during aligner treatment and, for the most part, have not found an explanation for this. A common site of this occurrence appears to be the maxillary lateral incisors in cases where these teeth are relatively intruded. This has been described in the literature as the “watermelon seed” effect. Since then, other data have emerged regarding the biomechanics within aligners that may be responsible for it. Accordingly, this review aims to evaluate the available evidence regarding this complication of aligner therapy and consider strategies to minimise or eliminate it.