While it is rare, your passport and visa may be spot checked by local police or officials, and if you have failed to register your visa you may be subject to on the spot fines of between 2,000 to 5,000 roubles. They may also take you to the local police station to check your details, passport and visa, and levy further fines.
If you have not registered your visa upon departure from Russia, passport control can fine you between 2,000 to 5,000 roubles. In some cases, they may immediately deport you, and you will be banned from entering Russia for five years.
If you receive two official fines, and your details are taken, these will be shared with Russian Consulates worldwide, and you may be barred from obtaining a Russian visa for five years.
If you have registered your visa, but simply lost your registration slip, you will not be liable for any fines. If you are still within the original registration period for your visit, you may be able to get a new registration slip, but you cannot get a replacement for the one you have lost.