Slip a cog is an idiom that means to make a mistake or to forget something. It is derived from the phrase 'slip a cog in the wheel, ' which refers to a gear slipping out of place and disrupting the normal functioning of a machine. The term ERR, which is a three-letter word, is an abbreviation for the verb 'to ERR, ' meaning to make a mistake or to do something wrong. Therefore, the crossword solutions for the term SLIP A COG can be summarized as 'to make a mistake' or 'to ERR.'