It usually means you have tried to turn the key to a different mode before you finished in the last mode. The machine is telling you that you haven't finished what you started. turn the key back to where it last was and finish the last transaction/ report/ programming etc.
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If anyone needs more information, the samsung/ Sam4s ER290 manual is here:
http://the-checkout-tech.com/manuals/cashregisters/Samsung_ER-290_operating_programming_manual.html
Registerman, February 2012
