I am not familiar with the POP CS bug in the 808x.  But in those early 
days, there were a few instructions that you did not execute in certain 
order.  One had something to do with PUSH or POP of the SP and the EI/DI 
instruction.  If an interrupt occured between these two instructions, the SP 
register was not properly saved.  There were some other 'hard to test for' 
bugs.  Ironic since Intel declared the 8086 as the first processor where 
every function performed correctly on the first prototype.
  All this while the 8085 was being designed simultaneously and elsewhere.  
The 8085 was the upgrade to the 8080 - Zilog was eating the 8080 sales with 
the Z-80.  The 8080 required a -5V supply.  The Z-80 only required a single 
supply - +5V.  The 8085 was Intel's answer to the Z-80, but some instructions 
failed to operate properly.  Those instructions therefore are called 
undocumented instructions.  Intel decided that it was more important to sell 
the 8085 as is - a fully function uProcessor - rather than attempt to fix a 
few unimportant instructions.
  As we know today, all the attention given to the 8085 and Z-80 was not 
significant since the real future was in the 8086 that was under design at 
the same time.
--- Maximus 3.01
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