[buildbot]

It seems way better to fail with a RuntimeException than to get possibly unsorted results that cause some hideous error deeper in your application.

[eru]

It depends on the contract.

It's good to have a contract that defines behaviour even in the case of invalid input.

That could be throwing an error, or it could be given unsorted input. Either way is ok.

(The C and C++ answer is to leave the behaviour in the face of invalid input undefined. Undefined means that anything goes, including formatting your hard disk.)