Excerpt from http://hbase.apache.org/book.html. If you google "hbase LeaseException", this page may not be on the first page.
12.5.2.
12.5.2. LeaseException when calling Scanner.next
In some situations clients that fetch data from a RegionServer get a LeaseException instead of the usual Section 12.5.1, “ScannerTimeoutException or UnknownScannerException”. Usually the source of the exception is
org.apache.hadoop.hbase.regionserver.Leases.removeLease(Leases.java:230) (line number may vary). It tends to happen in the context of a slow/freezing RegionServer#next call. It can be prevented by having hbase.rpc.timeout > hbase.regionserver.lease.period. Harsh J investigated the issue as part of the mailing list thread HBase, mail # user - Lease does not exist exceptions