I am experiencing this bug in version 0.22 as well.
It happens any time I turn on the following config in my job:
If I remove this setting, the bug goes away.
When this option is turned on, the following symptoms are exhibited:
1. During the reduce phase many of the reduce tasks will quickly read > 100%. The Job will then report 100% complete.
2. I have on occasion seen it then go back below 100% on the overall progress. It will oscillate between different numbers in that case.
3. The job appears to complete normally in spite of the progress reporting issues.