Can anyone explain what the macro MEM_WRAP_CHECK_1 is checking for?

And how calling the callback() sub below, with the commented out line enabled, would cause that macro to be invoked, and fail, producing the message "panic: memory wrap at...", but not when the line is disabled? 

SV *g_rec = 0;

int setCallback( SV* rec ) {
    g_rec =  rec; 
    SvREFCNT_inc( g_rec );
    return 0;
}

void callback() {
//    call_sv( g_rec, 0);
}

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In reply to panic: memory wrap by BrowserUk

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