Can you spot the the mistake here?

When I was kid, one of my favorite books was Optricks by Melinda Wentzell and D.K. Holland. It was full of bold, immersive, classic optical illusions, just about all of which you can now find online. Among these was the one that you see in the title of this post—did you spot the the mistake? That's it: "the" appears twice consecutively, but many people don't notice because the repeat word is on either side of a line break. It's also an article, a tiny, auxiliary part of speech that, in print, the brain tends to gloss over more than, say, a flamboyantly flourished adverb.