From what I can tell, there are two types of prop_physics in Portal. The first ones are the normal ones we all know: Cubes, Radios etc. The other ones are mice, cup shards, keyboards, and bean cans. They often come from a normal prop breaking; they are often wooden debris from a broken box. The best way to identify these types of props are from their collision properties. These objects do not collide with players. To avoid confusion, I’ll call them “Debris” (It’s hard to tell which category to place cameras into so I won’t.)
These different types of props have different types of collision properties. Debris don’t collide with other Debris or the player. Debris do collide with normal props and walls. Normal Props collide with everything. Cameras, however, are just weird. As we all know, cameras collide with players in a weird way known as Camera Bug and Camera Fly. Cameras collide with normal props in a normal way. But the collision between cameras and Debris are hard to describe but I’ll do my best. Cameras cannot push Debris. Debris can push cameras. Cameras cannot be stacked on top of an Debris. An Debris can be stacked on top of a camera. When save-loading while an Debris is on top of a camera, the Debris loses all collision with everything until interupted by a player or portal.
Now for something a little more useful, but still not useful. Debris are able to survive a fizzler. To do this, you must make sure that the last thing the Debris touched BEFORE going through a fizzler is a normal prop. The only use I have found for this glitch is preserving the coffee mug through the entire game. We call it Cup%. There is a speedrun of this by Kojlax if you’d like to check it out. Cup% is the only use I have found for this so far. I was hoping for something more useful, but players cannot collide with Debris which means no wallclimbing or object boosting with these.
The last glitch I noticed has no use so far, but is the coolest one so far. It allows for any object to be clipped through a wall at ease. You simply place a normal prop on top of a Debris and push it downwards and towards a wall. It will take a while, but you will find the prop to be pushed through the wall. I don’t see any use for this in Portal because the Portal Gun exists, but it could be something fun to do in Half-Life 2 while stuck in those boring cutscenes.
tl;dr: Debris have weird collision properties. You can get them to survive a fizzler and force normal props through walls easily.
The closest thing to a “good use” for these glitches is Cup%. If you find any good uses for these glitches please let me know what you’ve found. These types of props have always been weird to me and they just keep getting weirder.