> 1-indexed in libraries copied from Lua, 0-indexed elsewhere. That's just about the only worse thing you could do than 1-indexing everywhere

I'll do you one worse: in APL, you can set the index origin to 1 or 0 at will using the special variable ⎕IO. APL also uses dynamic scoping, so if you're in the habit of passing functions around, zaniness will result. (Thankfully this is not common.)

There was even a practice at one point of writing code that would work correctly with any value of ⎕IO.