If you
load up a world with default (not flat) terrain generation, and run

/kill @e[rm=1,c=20]

You would expect
20 things excluding yourself to be killed; the usual suspects: squid, bats, cows, pigs, sheep – and if you keep running the command: their drops.

What actually happens
That does happen, but, the chat reports some weird things being killed from time to time (if you do it enough), things I don't know how it's killing: item.tile.rail, item.tile.gravel

these screenshots were taken from -40 66 459, from seed 123.