A few years ago, I went to get a replacement bass wheel: mine had worn through and would no longer hold air after about 10 years of faithful service. (For those who may not know, this is what I'm talking about)
After some Googling, it turns out that the rather unusual tire size is quite common for medical equipment (like mobility scooters). So I went to the local medical supply store to see if I could save the wait/shipping costs/etc. of ordering online. I brought my old wheel with me just in case.
I asked the guy behind the counter at the medical supply store if he carried wheels like this--the size, etc. that I needed--and I showed him my wheel. He asked what it was for, and I did my best to explain that it was for transporting a musical instrument, like a big cello. His response, "Well, we do have wheels like that, but they're for medical equipment." I asked if I could see one of them, and he was hesitant, as if he couldn't understand why I was pursuing this further. After a minute, he went into the back and emerged a few minutes later with exactly what I was looking for--the right size and style, and it was a solid wheel, to boot. Perfect!
"But we only sell medical equipment."
"It's a wheel, and it's the exact thing I need. I'll take it."
"But it's not for medical use."
"It's a wheel."
Finally, he agreed to sell me the wheel. (Mind you, he was never gruff or irritated or anything throughout all of this: just perplexed.) I pulled out my debit card to pay for it...
"Do you have Medicare or Medicaid, or some other insurance we can bill?"
"Um, no. It's for a musical instrument. Can I just pay for it?" (I may have even brought cash along--something I don't typically do--anticipating some such issue.)
"Well, I'm going to have to call our billing department so that they can take your credit card information because we can't do that here."
It took three employees (including the person they called) and me about half an hour to resolve this transaction. The reason I like telling the story is because it's a great example of only being able to see (or hear) what you want to hear, what you're trained to hear. Seems to me a bit like a case of the duck-rabbit illusion in which someone can only see the duck.