A little failsafe feature in 3-prong electrical plugs
I happened to notice this today: if you go look at your garden variety three prong power plugs, you'll see that the ground connector is somewhat longer than the two power connectors. Thus when you insert the plug the ground wire is connected before the power goes hot, even if things are a bit off-balance.
There's applications of this safe-before-engagement features in system administration; for example, Unix boot sequences usually configure IP filtering before the network interfaces come up, not after. You can probably think of others.
(Disclaimer: example not necessarily valid outside of North America.)