When trying to root cause why
MAPREDUCE-4782 did not cause us issues on 1.0.2, I found out that HADOOP-7823 introduced essentially the exact same error into org.apache.hadoop.mapred.lib.NLineInputFormat.
In 1.X org.apache.hadoop.mapred.lib.NLineInputFormat and org.apache.hadoop.mapreduce.lib.input.NLineInputFormat are separate implementations. The latter had an off by one error in it until
MAPREDUCE-4782 fixed it. The former had no error in it until HADOOP-7823 introduced it in 1.1 and MAPREDUCE-375 combined the implementations together but picked the implementation with the off by one error in 0.21.
I will attach a patch that exposes the error.