File it with the footgun of the two different array slicing syntaxes: https://c3-lang.org/language-common/arrays/#slicing-arrays
I have already opened a discussion about this with the author, and I must say I agree to disagree that a language needs arr[start..end] (inclusive) as well as arr[start:len] (up to len-1) and if you use the wrong one, you’ve now lost a foot and your memory is corrupted.