That would require a catastrophic failure in the power brick which has significant separation designed in to the board layout to keep the primary line voltage side away from the secondary low voltage.

See this comparison teardown between a genuine Apple magsafe charger and a counterfeit, especially the "What's wrong with this charger" section: http://www.righto.com/2016/03/counterfeit-macbook-charger-te...

Related, Apple ran a discounted trade-in program where people could swap potentially dangerous phone chargers for safe ones after a counterfeit charger killed somebody: http://www.cbc.ca/news/technology/apple-replacing-fake-iphon...