Mazen Albeldawi and Rohit Makkar of the Cleveland Clinic bring us a free image in the New England Journal of what the bronchial tree looks like when a barium swallow test goes horribly awry. This patient had a very poor outcome, particularly unfortunate since this was an elective test.
The New England Journal also ran a previous image of barium aspiration back in 2003.
Barium swallow tests are performed safely innumerable times daily, and barium's inert properties are thought to make aspiration of small amounts safe. Yet, catastrophic complications can result if a patient aspirates a large amount of barium. No one knows how often this occurs, but several cases have been reported (and surely many, many more that have gone unreported):
The American College of Radiology has a practice guideline on the performance of barium swallow. The main safety point they make that may be more important than commonly realized is "[t]he patient must have cognitive awareness sufficient to cooperate with the study." Of course, since cognitive impairment and swallowing dysfunction often go together, determining who has "sufficient cognitive awareness" to avoid a serious aspiration event is difficult, to say the least.