Occasionally, it is unwanted for users to be able to send Ether to a smart contract. Unfortunately for these circumstances, it's possible to bypass a contract fallback function and forcibly send Ether.
contract Vulnerable {
function () payable {
revert();
}
function somethingBad() {
require(this.balance > 0);
// Do something bad
}
}
Though it seems like any transaction to the Vulnerable contract should be reverted, there are actually a couple ways to forcibly send Ether.
The first method is to call the selfdestruct method on a contract with the Vulnerable contract address set as the beneficiary. This works because selfdestruct will not trigger the fallback function.
Another method is to precompute a contract's address and send Ether to the address before the contract is even deployed. Surprisingly enough, this is possible.
Example from: https://consensys.github.io/smart-contract-best-practices/attacks/force-feeding/