Background Despite the high prevalence of locally advanced head and neck cancer, treatment failure in the form of a cutaneous deposit in the treatment field during radiation is not common. There has never been a cytology-proven published case of marginal treatment failure in the cutaneous region during radiotherapy. Case presentation A 51-year-old male patient was diagnosed with squamous cell carcinoma of the left tonsillar fossa. After a partial response to induction chemotherapy, the patient was treated with definitive chemo-radiotherapy. After the 23rd fractionation of radiotherapy, there was a clinical progression in the form of a solitary skin nodule within the treatment field, which was further treated with an electron boost to a total dose of 70 Gy followed by palliative chemotherapy. Conclusion During definitive chemoradiotherapy, failure outside the high-dose radiation field is not common, and a skin nodule during treatment had never been described. Our case demonstrates the importance of performing thorough clinical examinations on a weekly basis, not only for toxicity assessment but also for treatment response.