It's common knowledge that they put the Companion Cube in Portal in order to make people carry the cube through that whole stage instead of leaving it behind. What most people don't know is that isn't the only thing the developers added due to beta-tester behavior.
I'm speaking, of course, of the so-called "rattman dens:" certain, out of the way areas in the game where it looks to the player as if a previous runner through GLaDOS' maze healed up and began living again. The dens mainly consisted of photographs and insane message, scribbled over and over. These were added late in development.
The areas themselves were initially nothing more than small, barely bits of textured space that players managed to get themselves into with a little fiddling. Once these small, unremarkable spaces of map were reached, the beta testers who got there started acting odd.
The first man they found was sitting on the floor, curled up in the fetal position over his controller, rocking back and forth and sobbing quietly to himself. When they asked him what was wrong, he started shouting nonsense and tried to attack the guard who escorted him out of the building.
The next guy, well, actually a woman, went a bit further. Security rushed into the room when they heard screaming. The woman had clawed deep ashes into her own arms and was bleeding on the floor, twitching violently. They rushed her to the hospital, where she recovered fully without memory of the incident.
The third guy just stopped. He stopped playing, stopped moving, even stopped speaking. He had gone completely catatonic. His family apparently trid to sue for damages, but they didn't have a strong enough case. The developers, by this time, 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 beta tester seemed fine, the developers only found out what had 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; he had written the word 'cake' all over her body in permanent marker, also.
It IS noteworthy, though, that the Companion Cube as we know it was added later, after the beta test. All participants have shown the same odd behavior; at first catatonic and flegmatic, they became more and more violent with time.
A later symptom developed, too: an extreme sense of pyromania. Whether oven, campfire, or just a match, it always resulted in getting too close to the fire and being burned. In the end, Valve figured out a way to counter this behavior.
While originally, in the beta test, you simply left the cube in an air vent and moved on to the next level, now you have to burn the cube. You have to burn your best friend. Although they sealed off the air vent, it's still there in the level. It's just hiding under a wall of pixels.
Guess what happens if you find the air vent and don't burn the Companion Cube?