Background: Inverted left atrial appendage (ILAA) is a rare condition following cardiac surgery. Failure to recognize the condition or making misdiagnosis of a tumour, a thrombus or vegetation can lead to unnecessary and potentially adverse events. We present a case of ILAA after surgical repair of an atrial septal defect (ASD) in a young female. Case summary: A 3-year-old caucasian female was admitted for surgical repair of an ASD. The intraoperative course was uneventful until the opening of the right atrium (RA) after the commencement of cardiopulmonary bypass (CPB) and vacuum application, where the inferior vena cava (IVC) cannula was seen displaced in the RA. Cannula was repositioned, and ASD was repaired. On post-CPB transesohageal echocardiography (TEE), a newly developed mass was revealed in the left atrium (LA). The heart was re-arrested, and LA was re-assessed with unexpected finding of ILAA. ILAA was everted. RA was closed and CPB weaned off. Repeated post-CPB TEE showed no mass in the LA. No recurrence of mass was demonstrated on follow-up transthoracic echocardiography (TTE). Discussion: The incidence of ILAA is rare. Therefore, it is usually forgotten and not anticipated as a complication during heart surgery using CPB. In our case, dislodgement of the IVC cannula into the RA in combination with vacuum application in the setting of an ASD may have resulted in ILAA. This has not been reported in previous cases. ILAA should be suspected on intraoperative TEE if the mass is newly developed. Visual inspection of the left atrium appendage (LAA) in situ is recommended before chest closure.