A Tiny Toe Broke Half-Life 2’s VR Build

Former Valve developer Tom Forsyth has shared a memorable incident from 2013, when the studio was experimenting with a VR version of Half-Life 2. What started as a routine test quickly turned into a surprisingly serious bug—one that was both amusing and game-breaking.

According to Forsyth’s post on Mastodon, the VR build would softlock within minutes of starting. In a familiar early scene, a guard is supposed to open a door and let the player through. Instead, the door refused to move, halting the entire scripted sequence and leaving players stuck.

Forsyth called in additional developers, including members of the original Half-Life 2 team, to investigate the issue. It soon became clear that VR wasn’t the culprit. The same bug appeared even in the non-VR version when it was recompiled using a modern compiler.

The real cause was surprisingly intricate. Behind the door stands another guard, and in the original game, his position was calculated with extremely fine precision. When the door begins to open, it lightly bumps him, triggering a tiny rotation. In the old build, that subtle movement was just enough for his boot to move out of the door’s path, allowing the animation to continue smoothly.

However, the updated build produced a slightly smaller rotation—so slight that the guard’s toe still clipped into the door’s path. The door collided with this minuscule obstruction, stopped moving, and the game’s script stalled indefinitely. Players were left waiting for an event that would never occur.

The root of the problem was entirely technical. Early versions of Half-Life 2 relied on x87 CPU floating-point calculations, which handled precision differently. Modern compilers use SSE instructions, and those tiny mathematical differences were enough to alter the behavior of the game’s physics, breaking a sequence that originally relied on near-perfect alignment.