It's common knowledge that they put the Companion Cube in "Portal" to make players carry the cube through that whole stage instead of leaving it behind. What most people don't know is that it isn't the only thing the developers added due to beta-tester behavior.
I'm speaking, of course, of the so-called "ratman dens": certain, out-of-the-way areas in the game where it looks to the player as if the previous runner through GLaDOS's tests holed up and began living life anew. The dens mainly consist of pictures and insane messages, scribbled over and over again, mainly "The cake is a lie". These were added late in development.
The areas were initially nothing more than small, barely textured bits of space that playtesters would get themselves into with a little fiddling. Once these small, unremarkable spaces were reached, the beta-testers who got there would begin acting odd.
The first man they found was sitting on the floor, curled up in a fetal position over his controller. When asked what the matter was, he started shouting nonsense and tried to kill the guard who escorted him out of the building.
The next person, a woman, took it a step further. Security rushed into the room when they heard screaming. The tester had cut deep gashes into her arms and was bleeding on the floor, twitching violently. They took her to the hospital, where she made a full recovery without memory of the incident.
The third person just stopped. He stopped playing, stopped moving, even stopped speaking. He went completely catatonic. His family tried to sue the company, but they didn't have a strong enough case. The developers, by this point, were obviously very concerned, but what could they do? Video games don't drive people crazy.
It took a murder for them to change the areas.
Although the last tester seemed fine, the developers only found out about what happened when they got a call from the police. The last tester had forced his youngest daughter into an oven and burned her to death; the word "cake" was written all over her charred corpse in permanent marker.
It IS worth noting, however, that the Companion Cube as we know it was added later, after the beta test. All testers have exhibited the same odd behavior; at first catatonic, then fledgmatic, and then increasingly more and more violent over time.
A later symptom developed as well: an extreme sense of pyromania. Whether oven, campfire, or a simple match, it always led to getting too close to fire and burning. In the end, Valve discovered a way to prevent these things.
While originally, in the beta, you left the cube in an air vent and moved on to the next chamber, now, you must burn the cube. You burn your friend. The vent is still there, though. Hiding, under a wall of pixels, waiting to be rediscovered.
Guess what happens if you find the vent, and DON'T burn the Companion Cube?