Paper - No way. It will char it and make it go black.
Glass - yep. Clean first as AJ suggested
Plastic Credit Card - NO! Will fuck up the plastic if you're not careful. Inside of beer bottle top is probably OK if desperate, as it is lined with a plastic coating.
Metal - Should always avoid, as it can interfere with the result. Will also eat through the metal, and people often 'line up' on those surfaces, so please don't do it here!
Clothing - well, should go without saying, but this stuff WILL eat through most things, and you probably won't notice until you've put it through the wash!!
Skin - S**T man!! Wash the thing QUICKLY. This stuff WILL burn your skin, make it peel for a few days, and if you accidentally brush your contaminated hand on your mucous membranes (mouth, eyes, nose etc) you will BURN LIKE HELL!!
PLEASE - BE CAREFULL!! Not to be casually thrown around. This is basically battery acid!
If you lack a sensible area: In the toilet cubicle, lift the lid on the toilet. Wipe the rim of the porcelain bowl with toilet paper, and do it on the rim of the porcelain bowl. NOT ON THE SEAT!!
Also - PLEASE clean it up by wiping down with toilet paper and flushing! I'd hate to think of someone getting a severe chemical burn on their botty of fanny/willy because some dipshit left some somehwere in the toilet!
BE RESPONSIBLE, or we may find that the constabulary will make it a reason to ban the product. It is in your hands!
B.
Chemical Generation.