!ERROR=21 - Invalid Line Number/Label
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Using a line number outside the range 1..65534.
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Attempting to edit a nonexistent line.
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Attempting to delete a nonexistent line by typing the line number in console mode.
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Attempting to append to a nonexistent line by using the colon (:) feature in console mode.
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Referencing to a nonexistent line label.
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The callback subroutine registered to handle an event is invalid.