It was fixed by Satoshi long ago, back when we used CVS I think.

The problem was how scripts were executed. They were concatenated together and then run as a single unit. The now obsolete OP_CODESEPARATOR was put between them to control what was hashed and what wasn't.

The obvious problem with that arrangement being that scriptSig ran first (it has to, to push the signatures onto the stack), so nothing stopped you setting a scriptSig to OP_RETURN and making the script evaluate to true, always. A pretty amazing oversight given the thought and care that went into Bitcoin generally, and its robustness since then.

The fix was to move to the current system whereby the two scripts are executed independently but sharing a stack, and it's only the return value of the scriptPubKey that matters.

The scripting system always struck me as a rather late addition to the design. Satoshi admitted as much when he said that he added it after encountering an explosion of special cases as he designed various types of contracts. The fact that there's an obvious bug in CHECKMULTISIG is more evidence of this part being a general rush job, along with Satoshis willingness to disable much of its functionality later with the IsStandard checks. Also the design of CHECKSIG is an obvious retrofit, it would have made far more sense to decompose it, and we never found a use case for 99% of the opcodes despite having successfully designed (redesigned?) all the contract types he ever mentioned.