Left 4 Dead (Valve, 2008) keeps one hidden number on each of its four players. Valve's Michael Booth laid it out in a 2009 talk, The AI Systems of Left 4 Dead: the number, intensity, goes up when you take damage, get knocked down or pulled off a ledge, or when a zombie dies near you, and it drains back toward zero, but not while a zombie is still on you.

The AI Director watches the highest of the four. In Build Up it sends zombies until that number crosses a peak, keeps it up for 3 to 5 seconds, then stops sending and lets the fight end. Then comes Relax: 30 to 45 seconds with no new mobs, specials or wanderers (or until the team has walked far enough toward the next safe room), and it builds again. Bosses aren't affected, and specials already out can still attack. Because the timing follows the players, the peaks land somewhere new every run. In Booth's words, the algorithm "adjusts pacing, not difficulty", and "Survivor Intensity estimation is crude, yet the resulting pacing works."

Why not just place the zombies by hand? Booth's slides say players are good at memorizing static locations and the game "degenerates into a race". Nonstop doesn't work either: "Constant, unchanging combat is fatiguing."

The town, the cubes and the numbers in this film are our own simplified version, built in code from the rules on Booth's slides; the scale, the peak line and the rates are ours. The fixed-spots and nonstop runs are our own demonstrations.

Which game's hidden rules should we cut open next?

Sources: Michael Booth, "The AI Systems of Left 4 Dead", Valve, AIIDE 2009 (slides, valvesoftware.com/en/publications).
Sounds: Apple iMovie sound effects.