Okay, I went back and checked and I was wrong. The first installment of John Oliver's new series was not longer than 30 minutes. It was exactly 30 minutes, almost to the second. The problem was that the first time it aired — when I first TiVoed it — it started thirty seconds late due to promos so it therefore finished 30 seconds late.
It apparently did run long…long enough that they omitted the end credits, which must have delighted the staff no end. My guess is it was supposed to start 30 seconds or so after the announced start time and finish 30-60 seconds before the announced start time of the following program. By omitting promos, subsequent airings just fit into the half-hour. I'm still going to pad my recordings of future episodes.