If someone manages, through either a bug or a mistake (self-destructing and sending funds to the contract), another user can claim the funds as their own. Measure the balance before and after, and use the difference, rather than measuring the total balance of the contract
There is 1 instance of this issue:
https://github.com/code-423n4/2022-05-alchemix/blob/de65c34c7b6e4e94662bf508e214dcbf327984f4/contracts-full/WETHGateway.sol#L70-L75
