"Yes, but things are different in the real world."
It's perhaps not the most patronizing thing you can say to someone, but it's close to being the most patronizing thing anyone has said to me. (And I am deeply aware, gentle reader, that it is a privilege for me to be able to say so).
When I was at school, I was told the real world was and would be different. After I got online, people reminded me that I needed to spend time in the real world. When I went to university, graduates in full-time employment assumed an air of superior knowledge and sagely informed me that the real world was a different place. When I started work and my colleagues found out I was into open source software, they were happy to tell me how that sort of thing just doesn't work in the real world. When later I started work at Canonical, more than one person would patiently explain to me that sooner or later the pressures of the real world would cause the company to collapse. Now, at Google, I occasionally hear people joke about how it's scary out there in the real world since you have to pay for your food out there.
There is exactly one real world. It is the world that we are all living, breathing, eating and sleeping in. We have never in our lives left the real world. Each and every day has been spent within its confines, and it will be so every day until we go to our graves. If it turns out that there is life after that, then that too will be as real as today.
Is it possible that the very fabric of the universe shifts and alters at the gates of academia? That friends in the schoolyard mean less than friends in the workplace? Is it really true that being a jerk to someone online is somehow less real than being a jerk to someone standing next to you? Perhaps the laws of physics are suspended at Google, allowing the chefs to conjure food from the ether.
No. It's very clear what people actually mean when they say "the real world". They mean "my world", and add to it the sheer effrontery of suggesting that only their experience and the experience of those like them has any validity whatsoever.
To tell someone that things are different in the real world is to deny them an authentic experience of reality. That which you have lived through is of less value than that which I have lived through. My concerns have weight, yours are ephemeral. My achievements are solid, yours are imagined. I have the boldness to face the truth and all of its harsh consequences, while you wrap yourself in the warm blanket of pleasant delusion. I matter, you do not.
I won't bear any grudge if you use the phrase to compare and contrast obviously imagined worlds to reality. For example, "in Tolkien's Middle Earth, the Silmarils and later the Rings of Power are central, and the lives of thousands turn on them. In the real world, jewellery is rarely that important". If you said that, we might go on to have a spirited discussion about the diamond industry, but I wouldn't object to your phrasing.
However, if you say something daft like "Well, all of this open source software is well and good but in the real world we have to think about money", then I'm afraid it's going to be difficult for us to converse further.