modBy takes any integer as an input and therefore it's possible (and quite frequent if you're running it on indexes) to attempt to divide by 0 which provides a runtime error:

The elm lsp/compiler provides no warning for this and it successfully compiles, which contradicts the 'no runtime exceptions' rule

Easy issue to replicate, just pass 0 in to modBy. I'm unsure if this is intentional behaviour or not but after a day of writing elm it caught me off guard to have my first runtime error