I work with distributed systems. Try producing correct code with table sharding, replication, cache, realtime updates (targeted/efficient), distributed processing, streaming in batches without missing any rows while the data is being updated live...

When I worked with blockchain before, that was another level because each node had to talk with thousands of other nodes; each running different versions of the code and they had to propagate messages throughout the network in a secure and scalable way without missing any nodes or reaching the same node twice (spam vulnerability) so your code is basically talking to different versions of itself which feels a bit like recursion, but harder and with a network between which adds latency and errors; and you can encounter tricky issues like message storms among others where the nodes keep bouncing and replicating a message between themselves. Or if any kind of propagated processing doesn't deduct tokens, then that's a massive spam vulnerability. Also, if the peer-discovery algorithm is structured, then the network is vulnerable to eclipse attacks. Also, most parts of the code had to be fully deterministic and idempotent or else some nodes could fork off the network. Stamping out all non-deterministic, non-idempotent logic is hard work, especially when you have to do it across multiple distinct nodes running on different machines operated by different people! Paradoxically, some parts of the code actually benefited from being non-deterministic like the P2P networking and peer discovery logic since that made it harder to game/carry out eclipse attacks. So you need deep, nuanced knowledge to get it right.