Back when I joined the team on Far Cry 5, the game was approaching the final stretch before shipping.
I had just finished work on Assassin’s Creed Origins and moved over to help polish the game, fix bugs, improve systems, and basically help wherever help was needed. I was mostly working on NPC activities at first, but once that wrapped up, I started touching a bit of everything: mission scripting, gameplay bugs, weird edge cases, random polish tasks.
Classic end-of-production chaos.
One unlucky cult leader
One day, someone from Quality Control came to me with a particularly nasty bug in a side quest called Patriot Acts.
The mission was simple: the player follows one of the cult leaders to a house. As the player approaches, the NPC exits the house, gets into a helicopter, and flies away while the player chases him using an ATV.
The mission had a few fail conditions: kill the target, get detected, or lose the target. Pretty standard stuff.
Except sometimes, for absolutely no reason, the NPC would just die in front of the helicopter before even taking off. Mission failed. No gunshots. No explosion. No animal attack. Nothing. The NPC would simply collapse dead next to the chopper, and the repro rate was around 10%… just enough to ruin your day.
“Any repro?” Not really. “Any callstack, logs, anything?” Not yet, working on it.
Finding the murderer
So I loaded the mission on my machine.
Approach the house. NPC exits. Gets into helicopter. Flies away. Everything fine.
Second attempt. Nothing. Third attempt. Nothing. Twentieth attempt. Still nothing.
At this point I started wondering if QC had accidentally discovered a paranormal version of the mission. So I went back to the tester and asked him to reproduce it in front of me.
A few attempts later, it happened. The NPC died right in front of us.
My first theory was the helicopter itself. Maybe the rotor collision was misplaced and occasionally clipping the NPC? That seemed plausible enough. Back to my desk. Debug collisions. Check the helicopter. Nothing. Everything looked perfectly fine.
Back to QC. More attempts.
And then I noticed something important that nobody had mentioned. The tester was not running the game on PC, he was testing on a PS4 devkit, tucked behind his monitor.
Now things got interesting. This was still late production. Performance was not exactly perfect yet on console.
And suddenly a thought appeared: could framerate actually be causing the bug?
Back to my PC. I did everything possible to destroy the framerate: debug tools, terrible settings, heavy loads, whatever I could think of. Then I retried the mission.
Approach the house. Watch the NPC.
Boom. Dead.
Now we had a repro.
I started digging into the damage logs and confirmed something weird: the NPC was indeed receiving damage. A lot of damage. Enough for an instant kill. But from what? The collision name showing in the logs was vague and useless.
Physics had other plans
So I enabled collision rendering and started going frame by frame. And then I saw it: a tiny square, flying through the air, directly into the NPC’s head.
What the hell was that thing?
After tracing it back, I finally found the culprit. When leaving the house, the NPC opened a door. That door had a small window. And for some reason, the window had its own collision object attached to it.
At normal framerate, everything behaved correctly. The collision simply followed the moving door. But at very low framerate? Physics completely lost its mind.
The NPC slammed the door open, the door oscillated back and forth violently, and eventually the tiny collision box attached to the window decided to become an autonomous projectile. It launched itself perfectly into the path of the NPC. Straight into his head. Instant death.
Don’t assume things, ask
The fix itself was trivial: remove that useless collider.
But the process of finding it took several days: reproducing a 10% bug, realizing it was happening on console, suspecting framerate, intentionally tanking performance, debugging invisible collisions frame by frame, and finally discovering that a tiny window collider had become a lethal missile.
To this day, it is still my favorite bug I ever worked on.
Because this is the kind of thing that reminds you how absurd game development can be. You spend weeks building complex AI systems, missions, streaming tech and cinematic sequences, and then a small square attached to a door window decides to assassinate an NPC because the framerate dropped too low.
And honestly? That stuff is part of why I love making games.