I am using a certain 7 item Likert scale in my research and have collected my data - however I've now realised that I left one of the items (Item 7) off my initial survey. I have since collected all the data and am now looking to analyse it so there is no possibility of collecting this information retrospectively. 

What can I do about this "Missing" data? 

I have looked into imputing the data for this item based on mean substitution (or multiple imputation?) - however I'm not sure that this is really 'missing' data in that the participant didn't skip the question, it was just never included.. 

Should I be looking to impute this data for item 7 so that the total score for this scale is then comparable to past research? or should I just use the data collected for the 6-items and note this as a limitation?