Not because it's actually useful (almost entirely the opposite, in fact), but because I keep finding slightly insane corners of the language. And also this would freak out a beginner despite doing nothing useful.
Anyway, today I learned that you can use this:
[1,2,3,4,5].method('inject').([],&->(x,y){x+[y]})
to return this:
[1,2,3,4,5]
I'm sorry.
EDIT
I've made it worse
%w(x y).method('inject').([],&->(x,y){x+[y]})
The method bit's a bit wordy, but we'll get there eventually.