The fun differences between the perfect world of theoretical and the realistic. Everyone thinks of computers as perfect - but it's not until you're asked to solve "How do you store decimals using only 0s and 1s?" does it start to click. Not as easy. It's why I'm hesitant to hire bootcampers into my roles. Bootcamps are great, and they get more people coding, but you don't learn that theory behind the scenes - you don't really know what the computer and operating systems are doing. For 90% of the time it doesn't matter, it's abstracted away - but that last 10% man, that can really fuck up an entire system.