Here's a refreshingly obvious -- in retrospect, as usual -- example of selection bias at work.
I often eat my breakfast at a nearby cafe, which offers many types of sandwiches based on three kinds of bread - wheat, rye, and some kind of low-calorie thingie. I always order rye. Sometimes they're out of rye bread, and I have to choose one of the other two; it's no tragedy, but it can be slightly annoying, as I really prefer rye bread to the other two, by far.
The other day, I caught myself wondering at the strange problem this cafe -- actually, the whole chain this location is a part of -- has with rye bread. I mean, they regularly, if not too frequently, run out of it, but the other two kinds are always there to replace it when that happens. So why don't they just bake more rye bread? (they bake their own bread on location).
I've been drinking coffee and eating sandwiches in cafes from this chain for about 10 years now, and they've always had this infrequent, but persistent rye bread problem. What would explain this? Could the managers of the chain have some kind of a strange rye bread aversion? Is it business ineptitude? I think it's only the final baking that happens in the cafes; maybe the bakery that prepares the loaves for everyone has no feedback loop from the cafes, so they produce the same amount of different kinds, not knowing that customers will prefer rye bread and it'll run out more quickly. Maybe it's something else?
So I spent a few minutes, while sitting in the cafe, idly thinking about this "rye bread problem", and what could be causing it. That the problem itself exists was completely obvious to me, and has been for a long time without thinking about it. And then the obvious hit me: of course, there's no "rye bread problem". I just never care when they're out of any other kind of bread. It's probable that simply due to inaccurate estimates or fluctuations, they'll run out of one kind or another every now and then; but the cases when wheat or low-calorie is unavailable aren't visible to me (well -- I probably hear about it when someone before me in the queue orders a sandwhich of one of these two kinds and it can't be made -- and immediately forget about it).
Since I reached that startling insight about a week ago, I've been paying more attention, and already noticed two occasions when some other kind of bread other than rye was missing.