The primary concern is whether an input marked with ANYONECANPAY, when paired with SIGHASH_ALL, presents a vulnerability or not. The theoretical scenario posited involves a fully signed transaction that includes at least one ANYONECANPAY input, which covers the amount of the inputs it signs for, even when combined with SIGHASH_ALL. The issue identified is the potential for an external party to modify such a transaction by removing other inputs and rebroadcasting it. Since the modified transaction would still fulfill the necessary conditions for validity, there's a risk that the outputs originally intended as silent payments could become invalidated through this process. This situation underscores a nuanced aspect of Bitcoin's transaction mechanism, highlighting the complexities involved in ensuring transaction integrity while providing flexibility in signing options.