doc

Author: zsxwing

docs/rdd-programming-guide.md

@@ -1465,6 +1465,10 @@ jsc.sc().register(myVectorAcc, "MyVectorAcc1");
 
 Note that, when programmers define their own type of AccumulatorV2, the resulting type can be different than that of the elements added.
 
+*Warning*: When a Spark task finishes, Spark will try to merge the accumulated updates in this task to an accumulator.
+If it fails, Spark will ignore the failure and still mark the task successful and continue to run other tasks. Hence,
+a buggy accumulator will not impact a Spark job but it will not get updated even if a Spark job is successful.
+
 </div>
 
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