The only one of those edge cases I can think of is in an exploit I wrote (<a href="http:&#x2F;&#x2F;www.openwall.com&#x2F;lists&#x2F;oss-security&#x2F;2015&#x2F;08&#x2F;04&#x2F;8" rel="nofollow">http:&#x2F;&#x2F;www.openwall.com&#x2F;lists&#x2F;oss-security&#x2F;2015&#x2F;08&#x2F;04&#x2F;8</a>).  This part need to work when interpreted as 32-bit or 64-bit code:<p><pre><code>  1: .byte 0xff, 0xca &#x2F;* decl %edx *&#x2F;
     jnz 1b
     mov %%ss, %%eax  &#x2F;* grab SS to display *&#x2F;
  
     &#x2F;* Did we enter CPL0? *&#x2F;
     mov %%cs, %%dx
     testw $3, %%dx
     jnz 2f

     &#x2F;* this part knows it&#x27;s 64-bit *&#x2F;

  2:
     &#x2F;* this part knows it&#x27;s 32-bit *&#x2F;
</code></pre>
The .byte thing is because 32-bit x86 allows two encodings for decl %edx, but the one-byte encoding got recycled for REX on 64-bit.