It finally happened. TiVo let me down.
Because we've had relatives in town, and because we took off for Orlando for the last weekend, I've had a lot of TV to catch up on. Not only that, I haven't deleted any episodes from this season's "24" yet (which worked out well because we were able to introduce my mother-in-law to it). Finally, I record most programs in "High" quality, not the lowest setting; so my TiVo will store about 38 hours, not 80 (as it's rated).
The upshot is that apparently I exceeded its memory, and it started to delete programs to make room for new ones to be recorded. Did it delete last week's "Alias" (which I hadn't watched yet)? No, it kept that. (I must have had it on "Save until I delete"). But it did delete the season finale of "The Amazing Race," which I hadn't watched (though Yahoo! spoiled it for me already).
What's most annoying is that the programs TiVo needed space to record were this week's "Alias" (blah) and "Mythbusters" (repeats about 20 times a month), as well as "Survivor" and "The Apprentice" (watch and delete).