Hmm interesting, thanks for the report! I'll see if I can set up some kind of client-side error tracker so I can investigate more easily in the future.

It's possible that it timed out when computing the result — it evaluates the answer in a web worker with a 2.5s timeout and execution is slower on mobile. I used to get timeouts more regularly, but it's fairly optimized now — it checks all of the test inputs and evaluates both the expected solution and user-provided solution in a single Python function.