One of the biggest things I see in junior engineers that I mentor (working in backend high throughput, low latency, distributed systems) is not working out all of the various failure modes your system will likely encounter.

Network partitions, primary database outage, caching layer outage, increased latency ... all of these things can throw a spanner in the works, but until you've experienced them (or had a strong mentor guide you) it's all abstract and difficult to see when the happy path is right there.

I've recently entirely re-architected a critical component, and part of this was defense in depth. Stuff is going to go wrong, so having a second or even third line of defense is important.