cacheclean system test fails intermittently

Same failure mode has been observed in multiple jobs:
S:cacheclean:Fri Feb 16 09:03:14 UTC 2018
T:cacheclean:1:A
A:System test cacheclean
I:check correctness of routine cache cleaning
I:only one tcp socket was used
I:reset and check that records are correctly cached initially
I:check flushing of the full cache
I:check flushing of individual nodes (interior node)
I:check flushing of individual nodes (leaf node, under the interior node)
I:check flushing of individual nodes (another leaf node, with both positive and negative cache entries)
I:check flushing a nonexistent name
I:check flushing of namespaces
I:check flushing a nonexistent namespace
I:check the number of cached records remaining
I:check the check that flushname of a partial match works.
I:check the number of cached records remaining
I:check flushtree clears adb correctly
I:failed
I:check expire option returned from master zone
I:check expire option returned from slave zone
I:exit status: 1
R:FAIL
E:cacheclean:Fri Feb 16 09:03:37 UTC 2018