According to this, when the scriptPubKey equals to:
OP_n (with n between 0 and 16, inclusive) followed by a direct push of exactly 2 to 40 bytes inclusive
... it denotes the start of SegWit validating process. SegWit validation process include the checking whether the scriptSig is an empty field. If it is, old nodes (non-segWit) will automatically accept this as a valid script since after the "concatenation" (I know it's not a real concatenation) the top-most element in the stack will be non-zero, while the segWit nodes will go into further checking.
So, my question is: what will happen if someone just add
OP_1 in scriptSig? I mean, for non-segWit nodes it would still be accepted as
a valid script since top-most element in the stack is non-zero, and for segWit nodes it will considered as invalid since scriptSig is not an empty field. Will this make some kind of fork? How is this resolved?
Additional question is what will happen if non-segWit nodes accept some "segWit" transaction as valid since top-most item is non-zero, and segWit on the other hand do not accept that transaction since something in its witness data is not okay? What is happening in this case, how is this resolved?