I’ve traced it down to the fix for GEODE-7727 while I was working on GEODE-6008 with Mark Hanson on Friday.
https://issues.apache.org/jira/browse/GEODE-7727 It’s a timing issue that seems to mostly hit tests using SSL but could show up anywhere. There’s a PR up with a fix for the problem: https://github.com/apache/geode/pull/4839
