There are several mimics of a pneumothorax. Occasionally a patient's skin can form a fold that mimics the lung edge. If you suspect this is the case in a patient without clinical signs of a pneumothorax then you should consider repeating the X-ray.
Occasionally a large bulla, such as in this patient, can mimic a pneumothorax.
It is essential that you treat these patients with caution. ALWAYS look to see if there is an old X-ray (the cheapest and quickest test in radiology). If appearances have not changed then you know you are dealing with a bulla. If in doubt a CT can be performed following discussion with a radiologist.
Do not insert a chest tube if you are in any doubt about the diagnosis on clinical or radiological grounds.