So I managed to pull this off without cheats, in singleplayer. I actually didn't expect this to happen, at all.
SPOILER ALERT

http://cloud.steampowered.com/ugc/54177 ... F302DDC68/

I had a portal placed on the conversion gel in the Wheatley fight, then the water came and put the conversion gel off, however it kept my portal. Which left me with a portal on a surface where you normally can't portal onto.

You will also notice during gameplay that there are places in the game where this kind of thing is more intended, i.e. conversion gel getting covered with bounce gel and your portal remaining.

If you portal an area with conversion gel, and the gel gets washed away, only areas not covered by the portal are clear of the gel. And when the portal is removed the gel underneath is also removed.

This gives the 'illusion' that you have portaled a metal/black surface.

iWork925 wrote:
If you portal an area with conversion gel, and the gel gets washed away, only areas not covered by the portal are clear of the gel. And when the portal is removed the gel underneath is also removed.

This gives the 'illusion' that you have portaled a metal/black surface.

It's not really an illusion because you can actually go through the portal.

I didn't say you couldn't

He didn't mean it was an illusion that the portal was there, he meant it was an illusion that you had shot a portal on a metallic surface.