For me, this seems to happen nearly every 4 months. It almost happened tonight as I was setting the timer to record Arrow on the CW. My local affiliate carries syndicated basketball on Weds, so the CW lineup gets pushed back 2 hours each week. I cycled thru the line inputs as the Mag started up and saw black screen. Luckily, the clock's time was a few minutes fast, and I was able to stop the recording, and direct dial the digital ch which brought the signal back in without having to do a rescan with so little time before the show started.
But a rescan will be needed afterwards, to insure further timer recording goes without hitches like this.