I quite literally have had similar experiences. And yes, sometimes it was because the order taker was being careless. But there were quite a few times I discovered it was a food preparer who was so trained into being an factory line worker, that they pretty much stopping thinking for themselves! That of course, didn't surprise me that much.
But what did surprise me was how often workers in these would retaliate in various ways when they recognized me b/c I had complained they got my order wrong previously.
For example I'd say, I wanted very little lettuce on my burger at BK or a sub at subway. What would I get? Enough lettuce to make a huge salad! When this sort of thing started happening, I first thought "maybe their just mis-hearing my order." So I wrote down my orders (and I would order the exact same thing each time I went in). Oddly, I'd get the same results. Again, I thought, "well maybe it's My handwriting." So I printed out my order after that. And in Subways case, I even faxed & emailed it, Same response.
So fed up with it, the next time I went in to BK with my printed order and ordered three burgers. I paid for it, waited until my order came and called the mgr over before opening up my order, and then I proceeded to collect all the lettuce together from each of the burgers on one of the wrappers and then politely handed the mgr my copy of the printed order I made previously and asked "Since this isn't the first time this has happened here and my earlier polite complaints seem to be not working. So this time, I want *you* to tell me what you want me to do here - Should I simply ask for a refund for all this excess lettuce that I got charged for, but didn't order? Or, do want me to simply refuse it like other customers have done previously and make you throw it all away and lose some of your profit in the process and insist you make me three new burgers?"
The look I got from the food preparers in back and the mgr back to them, if looks could kill, there would've been a nuclear event that day... The mgr gave me a refund, told me I could keep the current burgers and please have a seat while she has them make up another set of three immediately.
While I sat down, I could tell she was having more than a few heated words with the group in back, occasionally punctuated with phrases like 'shrinkage', 'loyal customers', with a louder 'what were you thinking?', and 'Do you want corporate to tell us we need to find a job somewhere else?'.
After wards she brought out with the new burgers, handed me a complimentary meal card and thanked me for being such a patient customer, and said that she hoped I would come again soon. That mgr at BK got it right.
Unfortunately, I can't say that for the mgr at Subway when I pointed it out similarly when they did the exactly same thing with ONE SUB - he merely shrugged his shoulders and asked the next person what their order was.
That's fine and I'm sure they'll give me free dinner for lifetime in place of apology 
. I personally blame Manager for no "sensitivity training". It's clearly noticeable with different employees with same manager. 
they're still here!! Nice people with great service.
