Heuristics are great except when they don't work.

Today I was going to drop off some tea and cake for one of my wife's friends. Easy assignment: drive, pick it up from HITEA, drive, drop it off. There's no way I could mess that up.

I think I might have killed the suspense already.

There were three cups and three cakes in the bag—two mango cakes and one green tea cake, and two grapefruit teas and one milk tea. My job was to drop off one mango cake and one grapefruit tea.

I didn't know it, but my wife changed her call-in order just a bit after I left: the two grapefruit teas would be different—one smoothie, one regular tea. She sent me a message, but I missed it. I applied a really simple heuristic: I would look at the cakes and grab one of the two that looked the same, and likewise for the tea. One tea was in a cup with a dome lid, so I grabbed one of the two flat-lidded cups and dropped it off. Of course I dropped off the milk tea—the domed-lid drink was the newly added smoothie.

This was avoidable, even without close inspection of the contents:

There is no lesson to give you because I've never learned it myself. Maybe: avoid applying heuristics to things you don't know well, e.g., when foraging for food in the woods and trying something new, try an extra small taste and wait a while to see what happens, then try a small bite and wait a while, etc. Some of those edible plants and leaves and fruits look almost identical to poisonous varieties. Perhaps the better advice is: know the poisonous plants before you know the edible ones.

Or: even if it's something you know well, but it's risky (I'm thinking risk in the technical sense, in terms of likelihood and magnitude), use more than judgment. "We've launched this rocket on cold days before and it never crashed." Of course. It never crashes until it crashes, and then it's too late.

I hate being told what to do (it is my most American feature), but I welcome things like checklists at work. We can kill people if we have our own idea about what a "tight" fit is, or if we know the integration sequence for something because we've done a hundred times before, or because we thought it was third shift's job to assemble something. Simple things go badly wrong when you're hot, hungry, hungover, hurried, etc. Two cups might look the same, but only one is the right one.