This may be the most obvious piece of advice in the book:
|Prescription 40||When a formerly passing test fails, something has changed.|
Obvious or not, it’s worth repeating, mantralike, when confronted with a bad bug. When a formerly passing test fails, it means something changed.
It may be in the code, the system, or the test. But it’s probably not sunspots, and it’s probably not evil spirits possessing your MacBook (unless you’re living in one of Charles Stross’s Laundry novels).