shell> curl -X POST --data-binary @- --dump - http://localhost:8529/_api/transaction
{ 
  "collections" : { 
    "write" : "products" 
  }, 
  "action" : "function () {var db = require('internal').db;db.products.save({ _key: 'abc'});db.products.save({ _key: 'abc'});}" 
}

HTTP/1.1 400 Bad Request
content-type: application/json; charset=utf-8

{ 
  "exception" : "[ArangoError 1210: unique constraint violated]", 
  "stacktrace" : [ 
    "Error: unique constraint violated", 
    "  at Error (native)", 
    "  at eval (<anonymous>:1:99)", 
    "  at eval (<anonymous>:1:122)", 
    "  at post_api_transaction (js/actions/api-transaction.js:268:16)", 
    "  at Function.actions.defineHttp.callback (js/actions/api-transaction.js:288:11)", 
    "" 
  ], 
  "error" : true, 
  "code" : 400, 
  "errorNum" : 1210, 
  "errorMessage" : "unique constraint violated" 
}