The recent increase in the detection of papillary thyroid carcinoma (PTC) has been influenced by the finding of incidental tumours. To this group, carcinomas measuring less than 1 cm (the so-called microcarcinomas) as well as those above 1 cm belong. Analyzing a case series from our own experience, this paper focuses on the current pre-operative diagnostic challenges that can lead to PTC incidental discovery. For this retrospective study, 287 patients with a PTC diagnosis were selected. For each, the following variables were analysed: sex, age, ultrasound (US) appearance, number of thyroid nodules, PTC size, PTC variants and presence of other associated pathology. Pre-operative fine needle aspiration (FNA) results were classified according to the five-tiered SIAPEC system. For 281 patients, the US-guided FNA results were available. Cytohistological correlation was evaluated in terms of FNA sensitivity and false negative rate. An incidental PTC was found in 45.2 % of patients. The majority of these were due to unsuccessful US detection of malignant nodules (103 cases); incorrect cytological diagnosis was responsible for the other 24 cases. The most powerful clinical confounding factors were: multinodular background versus single nodule presentations (p < 0.001) and histotype (follicular vs conventional variant, p < 0.05). Of course, tumour size remains a strongly influential feature on pre-operative diagnosis, with greater difficulties arising for carcinomas <5 mm. Moreover, FNA sensitivity was lower also in large PTCs (>2 cm) due to tumour heterogeneity. Although with limitations related to the tumour's intrinsic features and the thyroid background, US-guided FNA, especially if performed by a dedicated multidisciplinary team, is a powerful diagnostic tool for detecting malignant thyroid nodules. To the state of the art, we propose a practical clinical-pathological cut-off for this procedure, setting it at 5 mm.