(For anyone who is like "that looks inherently undefined due to using i++ and i in the same statement", that is likely not true of Java anyway--which loves to define stuff like that--but more importantly: this issue turned out to have nothing to do with that, and is much more general and arguably a dangerous bug in the compiler starting with Java 9 related to +=.)
Yes this does look dangerous. Basically `strArray[keyLookup()] += “s”` evaluates `keyLookup()` or whatever is in the brackets twice. So if there’s a side effect to that function that’s going to behave badly. Limited to arrays of `String` apparently.