Unless they changed it. it is possible to have negative armor. so.. some of these calculations would result in you having negative armor and thus taking more than true damage
There are still a few sources of flat reduction in the game. Nasus spirit fire, Corki gatling gun, Taric shatter, etc. However, unless the enemy team is intentionally stacked to take advantage of this, you'll pretty much never be dropped below 0 anymore now that Black Cleaver doesn't have flat reduction.
(And if they do have a funny team which can give you -75 reduction or so, then just buy a single chain vest and you're safe).