Fasm should not allow input such as lock mov eax, [Sequence] because the lock prefix is not allowed in such a manner... Will be nice if there is some warning error instead of my application crashing because of unknown software exception.

LOCK is a prefix, and it's perfectly legal to use it infront of any instruction just like you could do REP MOV EAX,[Sequence]. I don't think fasm should report an error here. I'm sure tasm, nasm, masm, etc would, but that's why I use fasm. Warning sure, but fasm has no warnings.