Mastering Blockchain Programming with Solidity

Tokens can be locked

As per the ERC20 standard implementation you have seen, the transfer() method does not restrict sending the same tokens to the token contract address itself. This means sending TKN tokens to the TKN ERC20 contract itself. Hence, if the tokens are transferred to the contract address itself, those tokens will be locked forever because the contract code does not have a way to take the tokens out of the contract.

In the past, there have been many ERC20 contracts deployed where the tokens of the same contract are sent to the contract address itself; for example, Qtum (symbol: QTUM) and OmiseGo (symbol: OMG) tokens. Mostly, the tokens were sent by human mistake; however, it was the loss of the people who mistakenly sent tokens ...