What is this stuff?  That would easily be the question your asking yourself if  your visiting my miscellaneous folder.  So this is in the order that I've found items.

melt1.jpg and melt2.jpg are photos of a a melted 2x4 gray plate.  I found this is a tube of basic bricks I bought new.  The side was melted as you see in the photo.  I bought the tube on a very hot summer day and had to leave them in the hot car for over two hours.  So I don't know whether I melted it or it came that way.  Its the only melted brick I found in that tube.  I so amused by the discovery I kept rather than telling LEGO about it.

flame1.jpg and flame2.jpg are photos of a 1x6 gray plate with a black or dark gray swirl rear the center.  I can't tell you whether I bought new or used, but I'd guess used.  I spotted it while taking a break from build my cathedral.  I've tried to clean it but it's truly in the plastic.  

junk1.jpg and junk2.jpg are photos or a random bit of plastic I found with a new set.  The blueish-grey bit came with 4504 Millennium Falcon I got for Xmas '04.  At first I thought it was a broken piece.   But as put the set together there were no broken or damaged bricks to be found.  I assume its just an extra bit of ABS that found its way in.