I started to notice this about two months ago. A number of the little exchange boothes in the center have started leaving one ruble note stuck to the drawer through which they hand you your money.
What they do is this: you give 'em your dollars or euros, they pull out the right amount of rubles, and then count it for you (show you on a calculator and then put it in the drawer so you can get it out. When they put it in the drawer, they discreetly drop one of the notes on their side so that it's far enough in to be out of your vision. You, having seen them count out the money and put it in the drawer, take the money out and leave without counting it.
Lesson: ALWAYS count your money as you pull it out of the drawer. If it's not all there, put your hand back in all the way and see if there's a note hidden in the back.
The reason this one is so good is because it's nearly impossible to prove that they purposely do it. But I've tried two exchange points at Belorusskaya and they did the same thing three times in a row. I'm guessing that's why they have such good rates.