Unexpected server error when adding node to cluster (cluster 130 nodes)

When adding nodes to a cluster (using couchbase-cli server-add) occasionally (1-2%) get following failure:

failed: [ec2-52-16-246-187.eu-west-1.compute.amazonaws.com] => (item=ec2-52-17-4-153.eu-west-1.compute.amazonaws.com) =>

{"changed": true, "cmd": "/opt/couchbase/bin/couchbase-cli server-add -c ec2-52-16-246-187.eu-west-1.compute.amazonaws.com:8091 --user=Administrator --password=couchbase --server-add=ec2-52-17-4-153.eu-west-1.compute.amazonaws.com:8091 --server-add-username=Administrator --server-add-password=couchbase", "delta": "0:00:00.093626", "end": "2015-03-16 12:52:31.210082", "item": "ec2-52-17-4-153.eu-west-1.compute.amazonaws.com", "rc": 2, "start": "2015-03-16 12:52:31.116456", "warnings": []}

stdout: ERROR: unable to server-add ec2-52-17-4-153.eu-west-1.compute.amazonaws.com:8091 (400) Bad Request
[u'Prepare join failed. Got HTTP status 500 from REST call post to http://ec2-52-17-4-153.eu-west-1.compute.amazonaws.com:8091/engageCluster2. Body was: "\\"Unexpected server error, request logged.\\""']

This is repeatedly seen when creating a ~100 > node sized cluster.

( Setting as "feature-backlog". Plan is to capture issues found in large clusters as a precursor to supporting them.)