I recently had 66 on an ERJ flight. It was a simple DTW-EWR with no continuing flight out of EWR under the same flight number. I suspect it was because I was placed on an "earlier" flight that was delayed to depart at a similar time as my originally scheduled (also delayed) flight. I assume that many pax had previously been rebooked on various flights.

When requesting a seat change, I commented on this with the gate agent. She assured me that these numbers were random and had nothing to do with checking in. When I bet her lunch in Gordon's Galley that I was right, she responded with a very puzzled look .