Tim writes in his Retail Blog

Today I had a few spitters. They're actually fairly common; you may be one yourself. It's the people who lick their thumb or finger to separate two pieces of paper. This one woman was having trouble separating the credit card receipt after signing it so she stuck her thumb in her mouth and slicked it up with about an ounce of saliva. Then she used it to separate the two pieces of paper and, oblivious to the look of horror on my face, tried to hand me the store copy. No offense to anyone who does it but I'm not down with someone handing me their sopping wet receipt to file. It seems even worse to do it to money. I'll be standing there waiting for someone to count off a few bills from their wallet and suddenly the finger darts to the tongue, then back to the cash. Haven't they heard that four out of five bills have traces of cocaine, urine or feces on them? Small amounts to be sure, but I'm still not down with the moist art of the Finger-licking Paper-separation Maneuver.

Clink link to his blog entry