HP HP-21 - Error Display; Shift Key Cancel

Error Display
If you happen to key in an improper operation, the word Error
will appear in the display.
For example, try to divide 1 by 0 (the HP-21 will recognize this
as an illegal operation):
Press Display
| ENED
0
You can clear the error by pressing or by keying another
number into the displayed X-register.
Press Display
All those operations that cause the word to ap-
pear in the display are listed in appendix B.
Shift Key Cancel
If you press the blue shift key ## and wish to cancel before
another key is operated, press Ef&y and continue. For
example:
Press Display
2
Whoops! You wanted to , not [/ ], so:
Press Display
In this case, the normal EXiE function is not performed.
Pressing after the blue shift key [ does nothing but
cancel the shift key.