Over halfway into my shift I happen to glance up at the PI StripTools and noticed something amiss:  Mode27 was flat & elevated! 

Sure enough on VERBAL_ALARM, this mode rung up at 2:16utc & I didn't catch it until 5:13utc.  I immediately flipped the sign on its phase and this brought it down.  While it was elevated a few channels looked odd (Mode 24 was flat and lower for instance).  Attached is how these Modes & H1 Range look over this time.

Surprised H1 stayed locked over this time.  Surprised to see range approach 70Mpc toward end of elevated MODE27 ring up.