How do you make invisible bricks?

[aquaMat]

I actually do remember this (alas....I'm givin' it all away.. ):

I first added a second floor in layer 6 using

Fringe ON

. Later I removed that floor, but somehow (that's the part I really can't remember exactly anymore) removed it with

Fringe OFF

, theoretically leaving the fringe in the picture. Of course you CAN'T see

fringe only

.
Due to some bug in the software, you now step onto that floor, even if it is already gone....giving the impression you were walking on air.

After I stumbled upon this effect / bug accidentally a while earlier (where I had removed a single

layer-6 + fringe

brick), I tried to recreate it in TMF for an entire floor.

BTW, I also experimented with layer-3, but that led to nothing.