I updated a sheet by cutting/pasting from another, very similar sheet. Doing so broke the sheet. The problematic cell was this one:
=IF(category="Gin", gin_cost,IF(category="Whisky", whisky_cost, IF(category="Premium Whisky", premium_whisky_cost, rum_cost)))
gin_cost, whisky_cost, and so on are named ranges pointing at single cells elsewhere in the sheet, but after the cut and paste it seems like those named ranges were not know to the presently active document, despite most definitely being present.
I managed to fix this problem by editing the problematic cell. I changed gin_cost to the literal cell reference, B29, and then changed it back. This was enough to make the document reread its stored list of named ranges.