Chest pain with ST-segment elevation usually triggers immediate activation of myocardial infarction protocols. However, non-ischemic causes such as pericarditis may mimic STEMI, creating diagnostic and management dilemmas for primary care and emergency clinicians. This case illustrates the importance of clinical reassessment and continuity of care within the Spanish public healthcare system.A 65-year-old woman with dyslipidemia and hypothyroidism presented to her primary care center with acute chest pain. ECG revealed inferolateral ST elevation, leading to activation of the regional STEMI code and transfer to a coronary unit. She received aspirin and ticagrelor. On arrival, physical examination was normal. Repeat ECG confirmed inferolateral ST elevation, but troponin T remained normal. Echocardiography showed preserved LVEF without wall-motion abnormalities or pericardial effusion. Chest X-ray was unremarkable. Given the mismatch between ECG and biomarkers, cardiology revised the diagnosis to acute pericarditis. She was discharged on ibuprofen and colchicine with cardiology and primary care follow-up. Weeks later, the patient developed dyspnea. A chest radiograph showed a right-sided loculated pleural effusion with an air–fluid level, suggesting hydropneumothorax versus complicated effusion, along with bilateral parenchymal infiltrates. This required further evaluation and coordination between primary care, radiology, and hospital services.This case combines two unusual features: focal inferolateral ST elevation mimicking STEMI and a late loculated pleural effusion after pericarditis. Together, they demonstrate an uncommon diagnostic trajectory not frequently described in primary care settings.ST elevation does not always represent STEMI; clinicians should consider pericarditis when biomarkers and echocardiography do not support ischemia. Primary care follow-up is essential, as complications such as pleural effusion may arise weeks later. Improved communication between healthcare levels enhances diagnostic accuracy. Incorporating point-of-care ultrasound into primary care may facilitate earlier detection of pericardial and pleural involvement.This case highlights the risk of premature diagnostic closure in STEMI activations. Clinical evolution, normal troponin, and absence of structural abnormalities prompted reconsideration. The late pleural complication underscores the systemic inflammatory nature of pericarditis and the role of longitudinal primary care monitoring.This case underscores the importance of broad diagnostic thinking, interprofessional collaboration, and continuity of care in managing chest pain and its potential complications.