A nice case, and photos, of pneumopericardium from a perforated peptic ulcer. How, since the stomach is not in the mediastinum, you ask? You'll have to read the case -- it's free.
Andrianov A, Nissenbaum MA. Pneumopericardium Associated with a Peptic Ulcer. N Engl J Med 2011;365:2412. FREE FULL TEXT