Back in the archives there is another post on this same problem. My venerable -15C did the exact same thing this past week after I replaced its very dead batteries. No amount of button-pushing tricks would clear it. The USER flag remained on even when f USER was pressed and the top row function keys were operating in the inverted condition, per the USER function. Even ON y^x had no effect.
Removing the batteries initially did not work, and I noticed the memory stayed up even with the batteries out ten minutes or so. This little CMOS wonder is amazingly robust with no power. However, removing the batteries overnight did clear the fault.
I could not work up the courage to try the paper-clip shorting of the batteries suggested in the Users Manual.
I'm glad to have my personal copy of the finest pocket calulator ever built back in perfect operating condition.