The word 'dingbat' originally refers to to a character or symbol used in typesetting which gives an instruction to the printer. These should be removed before the text is published, but were occasionally left in by mistake. The character made no sense to non-typesetters and the association of "dingbat" with "nonsense" and "forgetfulness" carried over to describe people.
Julia's such a dingbat, I asked her to buy me yogurt and she got me cream.
This end of year report is riddled with inaccuracies. Looks like we've got a dingbat in the mix.