This is very obvious for the 10 rouble overprints on Postal Savings Bank stamps. Here the plates were prepared in such a way that if the plate was carefully applied the "10" obscured the 1, 5 or 10 of the underlying stamp and the "rublei"obscured the underlying "kopeka". See the top row of stamps in the illustration below.
Most forgeries make the mistake of placing the "10" centrally above the "rublei" so that the "10" never obscures the underlying 1, 5 or 10 - see the rest of the stamps below, all of which are forgeries.
Notice that on the basic stamps, the "1" is aligned to the left and the "5" and "10" to the right. As a result, the Ekaterinodar printer had to prepare two plates for the overprinting. The forgers economise and use just one. One of the forgeries (bottom row, third stamp from left) does realise that the "10" should be above the letter L of rublei, not the letter B, but the quality of the lettering is very poor in comparison to the genuine stamps.