Submitted by hansfriese, also found by rbserver
Total assets of yearn vault are not correct so the calculation between the asset and the shares will be wrong.
In YearnAdapter the total assets of current yValut are extracted using _yTotalAssets.
But in the yearn vault implementation, self.totalIdle is used instead of current balance.
In yearn valut, totalIdle is the tracked value of tokens, so it is same as vaults balance in most cases, but the balance can be larger due to an attack or others fault. Even sweep is implemented for the case in the vault implementation.
So the result of _yTotalAssets can be inflated than the correct total assets and the calculation between the asset and the shares will be incorrect.
Use yVault.totalIdle instead of balance.
RedVeil (Popcorn) confirmed 
