<p dir="auto">runtime/mbitmap.go says:</p>
<div class="snippet-clipboard-content notranslate position-relative overflow-auto" data-snippet-clipboard-copy-content="    // The following ensures that we are rigorous about what data
    // structures hold valid pointers.
    // TODO(rsc): Check if this still happens.
    if false {
        // Still happens sometimes. We don't know why.
        printlock()
        print(&quot;runtime:objectstart Span weird: p=&quot;, hex(p), &quot; k=&quot;, hex(k))
        if s == nil {
            print(&quot; s=nil\n&quot;)
        } else {
            print(&quot; s.start=&quot;, hex(s.start&lt;&lt;_PageShift), &quot; s.limit=&quot;, hex(s.limit), &quot; s.state=&quot;, s.state, &quot;\n&quot;)
        }
        printunlock()
        throw(&quot;objectstart: bad pointer in unexpected span&quot;)
    }"><pre class="notranslate"><code class="notranslate">    // The following ensures that we are rigorous about what data
    // structures hold valid pointers.
    // TODO(rsc): Check if this still happens.
    if false {
        // Still happens sometimes. We don't know why.
        printlock()
        print("runtime:objectstart Span weird: p=", hex(p), " k=", hex(k))
        if s == nil {
            print(" s=nil\n")
        } else {
            print(" s.start=", hex(s.start&lt;&lt;_PageShift), " s.limit=", hex(s.limit), " s.state=", s.state, "\n")
        }
        printunlock()
        throw("objectstart: bad pointer in unexpected span")
    }
</code></pre></div>
<p dir="auto">This needs to happen for Go 1.5.<br>
We broke a lot of programs in Go 1.4 by insisting on this<br>
(for good reasons), and we don't want relapses during Go 1.5.</p>
<p dir="auto">We used to have a check for pointers that point into the heap but don't point to an object.  This check is useful for debugging live variable info, global ptr maps, etc.  It is currently turned off.  We should re-enable it for 1.5.  It is currently broken at least because of holes in 32-bit heaps <a class="issue-link js-issue-link" data-error-text="Failed to load title" data-id="57681833" data-permission-text="Title is private" data-url="https://github.com/golang/go/issues/9872" data-hovercard-type="issue" data-hovercard-url="/golang/go/issues/9872/hovercard" href="https://github.com/golang/go/issues/9872">#9872</a> .  There are probably other reasons as well.</p>
<p dir="auto">The check is currently disabled in runtime/mbitmap.go:heapBitsForObject, line 181.</p>