If you log in onto machine and make an error, a machine is down.

If you put everything into configuration management, make scalable, redundant and interconnected servers and services built on top of it, you unlock amazing possibility of one error bringing hundreds if not thousands machines down.

Nice lil reminder that adding more things for redundancy only increases it if they are entirely unrelated to eachother and independent.

Which INCLUDES not running same software. You can have redundant switches, doesn't matter if they have software bug that triggers on all of them (or just one in a way that prevents network from working correctly).

You can have database cluster that can tolerate node dying just fine but still be brought down by a bug despise redundancy.