on_update_failure is non zero intermittently, in counter increment SBM test

We see on_update_failure intermittently on reruns of this particular test : 

Functions executed/sec, 1 bucket x 100M x 1KB, 4KV + 1Eventing node, Counter Increment 0% Contention Source bkt mutation 

We have seen this on 2 out of 3 reruns of the same test. So, it might be worth looking at

Failed runs : 

http://perf.jenkins.couchbase.com/job/themis/10745/ 

http://perf.jenkins.couchbase.com/job/themis/10742/ 

Passed run : 

http://perf.jenkins.couchbase.com/job/themis/10744/ 

Please note that the resident ratio on the source bucket is slightly less than 100% (at 95%) in these runs . We have not seen this behaviour in the 100% resident ratio test as of now.

on_update_failures : from the stats 

11:30:48 "on_delete_failure": 0,

11:30:48 "on_delete_success": 0,

11:30:48 "on_update_failure": 2,

11:30:48 "on_update_success": 158406056,

11:30:48 "processed_events_size": 201887797956,