(I work at Hackerrank) A couple points:

* the stack trace -- We avoid showing stacktraces to candidates as it has the potential of leaking information (a well created error message/stack can leak test case data).

We can and should show the trace (for languages that provide it) to reviewers. I've added an item to our internal tracker.

* Familiarity of environment: Most (if not all) of the challenges companies created by companies link to a sample test (hackerrank.com/tests/sample). We notice many but not all candidates take this before the actual challenge. Perhaps we should highlight this more.