Two key design characteristics of machine learning (ML) systems—their ever-improving nature, and learning-based emergent functional behavior—create a moving target, posing new challenges for authoring/maintaining functional regression tests. We identify four specific challenges and address them by developing a new general methodology to automatically author and maintain tests. In particular, we use the volume of production data to periodically refresh our large corpus of test inputs and expected outputs; we use perturbation of the data to obtain coverage-adequate tests; and we use clustering to help identify patterns of failures that are indicative of software bugs. We demonstrate our methodology on an ML-based context-aware Speller. Our coverage-adequate, approx. 1 million regression test cases, automatically authored and maintained for Speller (1) are virtually maintenance free, (2) detect a higher number of Speller failures than previous manually-curated tests, (3) have better coverage of previously unknown functional boundaries of the ML component, and (4) lend themselves to automatic failure triaging by clustering and prioritizing subcategories of tests with over-represented failures. We identify several systematic failure patterns which were due to previously undetected bugs in the Speller, e.g., (1) when the user misses the first letter in a short word, and (2) when the user mistakenly inserts a character in the last token of an address; these have since been fixed.