Long distance flying, happened to be over an ocean, when my elytra broke and vanished.  It had Mending I and Unbreaking III enchantments.  It did not give any durability warnings, rather just the breaking clunking sound after which I fell into the ocean.  When I checked for my elytra status, I found it was gone and no where to be found.  I am on a vanilla minecraft server with Default Game Mode set to Survival and Difficulty set to Normal.