Quick notice: I just found out that the current world record portal run actually uses this bug.

I was trying to practice using the portal mod for a potential speedrun attempt, when I found that if you place a portal, then press Q and F at the exact same time (I used a macro to test this), the blue portal (or possibly orange) starts to inch itself through the wall. If you then go through the orange portal, you just appear on the other side. I'm not sure if this works on the latest version because the drive link's dead so I couldn't download it (I had to install mod version 2.0), but the rules say that ANY version of the mod is allowed, so this bug might be allowed (I'll leave it to the mods to decide that).

Edit: I also found that your portals count as a character. For example, if your portal's on a button, it thinks you're on it, meaning portals can be used to trigger buttons. Also, your portal can be used to load and unload zones. For example, I accidentally shot my portal through the wall of the final room using this bug, and the ending dialogue started playing. I also found that doing this trick on the ceiling of the ending room actually triggers the secret ending dialogue, as if you actually went into the room.

The world record uses the earlier version of the mod.

And one way the "character" property is exploited is by shooting a portal down the final hallway to activate the end dialogue earlier