Okay I'm really confused by this. I had a spreadsheet set up which would give a dropdown list on one cell based on the values in another. I did this with conditional data validation using this formula:

=IF(D1="Building",Building,IF(D1="Mechanical",Mechanical,IF(D1="Electrical" ,Electrical, "Please enter Description")))

Not I wanted to change this because I needed more options in the initial cell so I changed the formula to this:

=IF(D1="Building",Building,IF(D1="Electrical",Building,IF(D1="Heating and Ventilation",Building,IF(D1="Air Conditioning",Building,IF(D1="Window Blinds",Building,"Please enter Description")))))

Now when I try and enter this it comes up with an error saying "The List Source must be a delimited list, or a reference to a single row or column".

What's even more strange is that I had a back up of the spreadsheet before I changed it and I deleted the formula in there and reentered it as exactly the same and got this error.

Does anyone have any ideas?