Mysterious lines appearing when printing a sheet

[Smeddlesboy]

I run a website with free spreadsheets that automatically generate worksheets for maths teachers, and have a recurring problem that I've been unable to solve for the last two years.

Basically the spreadsheets run using macros, various conditional formats. However when some particular spreadsheets are printed, a pattern of borders appear even though there are no borders around the cells in question, and no condition formats involve borders, so I am mystified as to how they can appear.

I've attached a sample file here, and have disabled protection so you can access all the code

If you press print preview, you should see the problem: it occurs to the left of column Z on rows 5, 6, 7 and 9

I'm betting its something ridiculously simple that I've just failed to spot, but I need somebody else's eyes to check it I think!