There are bold white horizontal lines appearing randomly in some of my
worksheets: where I have filled a group of cells in colour, they show up.
They look like cell borders but they aren't, nor are they drawing objects,
nor rows that have been reduced in height.

Deleting the rows above/below one of the lines does not get rid of it.
Deleting the columns that it spans does seem to make it disappear - however
that will cause problems for the contents of the worksheet.

Has anyone experienced this problem and how do I get rid of it?