Falsification of bills of lading causes the largest proportion of losses due to fraud. The importance of close controls on the preparation and transference of bills of lading arises from their service, not only as receipts, but as proofs of the transport contract and of ownership of the goods. A bogus bill may be sold along with forged accompanying documents to a buyer who may later find that the cargo and perhaps even the ship does not exist. Other major types of fraud include shipowner theft of cargo and/or fraud against insurance companies sometimes involving the scuttling of overinsured vessels in which case additional crimes are perpetrated, such as against personal property and life. Fraud is also committed by shipping agents billing customers for services never performed. There is also falsification of mortgage documents to enable intermediaries to pocket part of the mortgage repayments. Understated invoicing to avoid customs taxation or overstated invoicing to take foreign currency out of the buyer's country in violation of foreign exchange controls make governments victim of these maritime related crimes.