Like @EricL said, this would be caused by some process updating files in the table in the background without a refresh in Impala. E.g. if you have a job that writes files directly into the table and can either write incomplete files or has Impala see the files before they are completely written (preferably you write the files in a temporary directory then move them into the table directory). Some usage patterns for hive might cause issues, e.g. INSERT OVERWRITE. There was a related issue in Impala that could occur if you did an "INSERT OVERWRITE" from hive without a refresh from Impala: https://issues.apache.org/jira/browse/IMPALA-8561. Generally that workflow (insert overwrite without refresh) is problematic, but the symptoms were made more confusing by IMPALA-8561.
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