The last amended return I did was for a client who received a corrected statement for unemployment compensation.  It was for about $2,000 and they were in a 24% tax bracket, so the amount owed was about $480.  (No credits or other complications.)  They paid the $480 and just received a bill from IRS for another $450.  No explanation of the change.  I think IRS is catching up with the backlog with a new computer program that uses Artificial Stupidity.