ITBLL with 1.25B rows failed for me (and Stack as reported in https://issues.apache.org/jira/browse/HBASE-13811?focusedCommentId=14577834&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-14577834)
The root cause this time seems to be different. It is due to procedure based flush interrupting the flush request in case the procedure is cancelled from an exception elsewhere. This leaves the memstore snapshot intact without aborting the server. The next flush, then flushes the previous memstore with the current seqId (as opposed to seqId from the memstore snapshot). This creates an hfile with larger seqId than what its contents are. Previous behavior in 0.98 and 1.0 (I believe) is that after flush prepare and interruption / exception will cause RS abort.