Hmm. I was able to reproduce the behavior with your go program on my dev machine (C* 2.0.12). I was hoping it was going to just be an unchecked error from the .Exec() or .Scan(), but that is not the case for me.
The fact that the issue seems to happen on loop iteration 10, 100 and 1000 is pretty suspicious. I took a tcpdump to confirm that the gocql was in fact sending the "write 100" query and then on the next read Cassandra responded with "99". I'll be interested to see what the result of the jira ticket is. -psanford