Index: src/main/java/org/apache/hadoop/hbase/master/AssignmentManager.java =================================================================== --- src/main/java/org/apache/hadoop/hbase/master/AssignmentManager.java (revision 1432803) +++ src/main/java/org/apache/hadoop/hbase/master/AssignmentManager.java (working copy) @@ -1161,7 +1161,9 @@ } else if (rs.isSplitting()) { LOG.debug("Ephemeral node deleted. Found in SPLITTING state. " + "Removing from RIT " + rs.getRegion()); - this.regionsInTransition.remove(regionName); + synchronized(this.regionsInTransition) { + this.regionsInTransition.remove(regionName); + } } else { LOG.debug("The znode of region " + regionInfo.getRegionNameAsString() + " has been deleted.");