The Game's Afoot
The bug came up three times during the broadcast. After isolating the incidents where it happened, I looked for commonalities. Some we already knew, which had led us to dead-ends (it happened in the usual level against the usual faction). Fairly quickly I noticed another common thread - the bug always happened when time ran out in Attack mode.
So I went and tested for a while, running those same combats in that level, taking a variety of actions but always letting time run out. Still Nothing!
So I went back to the video for more clues. Though as a writer I am a noted Sherlockian no pipe-chewing, violin-playing, or Watson-shaming helped. I just stared at the screen over and over looking for the smallest detail. And small it was - I eventually noticed Jak's mouse move off to the corner of the screen each time. It was obscured by the Twitch interface, but could he be hitting the Attack button?
Eureka. I had found it. The bug occurred when you hit the Attack button at the same time the timer ran out. This would double-trigger the end of phase and cause the overlap. So obvious in retrospect.
Why Did We Miss It?
The most interesting part of this whole experience to me, and to other game devs I think, is the variety of red herrings.
1. Why Couldn't We Reproduce
Simple - we were too good at our own game. We either had our paths set with plenty of time left, or knew that it auto-committed your route when time ran out, so we would just let it go and use that last half second to keep planning in tight situations. It would never occur to us to rush to click Attack at the last second.
2. Why Did it Appear in That Level with Those Enemies
Gothic City is the third major level in the game. There is a prologue, then a level where you get comfortable with mechanics, then this one. So essentially, it's the level where the game starts to get hard. Before then it is teaching you, and after then you are more comfortable. But at this point, the patterns on Offense mode are, for the first time, hard to see. Thus you are more likely to scramble against time trying to find them.
3. Why Could No One Report
It happened when trying to click Attack as time ran out, perhaps the most stressful or focused part of gameplay. Of course no one would remember details of what happened when they were racing to beat the clock and survive.
Bug Fixed, Lessons Learned
Sam fixed it in no time, but we both found the experience extremely interesting and enlightening. An example of false perceptions and coincidences, making a mystery unsolvable by someone too close to it. Thankfully we lucked into the video evidence, did our detective work, and can move full Steam ahead.