The attached module, when compiled with -O2, has wrong code generated for the last set_global_field call. The assembly shows:

        leal    4(%edx), %eax
        cmpl    marked_limit, %eax
        movl    BadArguments, %ecx
        movl    $record_12420+4, (%edx)
        movl    %eax, marked_top
        jbe     .L28
        movl    %ecx, record_12420+4
        call    gc
.L28:
        leave
        xorl    %eax, %eax
        ret

while the assignment of BadArguments (from %ecx) to record_12420+4 should be performed unconditionally, as can be easily deduced from the source.

This is gcc-4.2.0-0.20060806r115974. The bug doesn't manifest without -O2 or when the number of set_global_field calls is reduced.