Cat
The domestic cat operates according to predictable, if inconvenient, principles. It will demand food at consistent intervals, deposit itself in warm locations with mathematical precision, and produce hairballs during moments of maximum social embarrassment. A cat's behaviour, whilst often inexplicable in specifics, follows discernible patterns: it will knock objects from surfaces, ignore expensive toys in favour of cardboard, and demonstrate sudden bursts of activity at precisely 3:47 AM. One can rely upon a cat to be reliably unreliable in thoroughly documented ways.
Luck
Luck distinguishes itself through spectacular unreliability, which proponents argue constitutes its essential nature. Good luck arrives without invitation and departs without notice. Bad luck appears precisely when one has forgotten to touch wood. The same actions that produced fortune yesterday yield catastrophe today—a variability that would constitute defective product design in any manufactured good. Gamblers speak of luck as though it were an entity with moods, grudges, and preferences, yet no amount of ritual has ever produced consistent results. Luck's only reliable quality is its refusal to be reliable.