Why shouldn't you inflate your life vest until you're outside the airplane? On commercial flights, in part of the pre-flight safety demonstration, you're told in the event of a crash, not to inflate your life vest until you're outside the airplane &mdash; why is this?
Ethiopian Airlines Flight 961 was the flight that resulted in this addition to the safety briefing.

After being hijacked, the plane eventually crash-landed in the ocean. Many passengers died because they inflated their life jackets in the cabin, causing them to be trapped inside by the rising water.

Mayday/Air Crash Investigation S03E13 (Ocean Landing) covered the incident. The relevant clip is here (from 23 seconds in):