Nice collection of “LEGO Abominations” – “These gender-bender bricks let you change the polarity of LEGO sculptures. These were made by carefully attaching pairs of bricks after wetting their interfaces with acetone.” – Link.
Also check out – “The “X-Ray brick” is a fully functional but inside-out Lego brick – It was made by attaching two regular 2 x 4 bricks together with acetone. The pair was left stacked with other bricks to maintain spacing while the acetone dried. Once the acetone had cured, the two side portions– effectively 1 x 4 bricks– were cut off of the sides, leaving the brick as you see it here.