Why is InDesign creating strange patterns in several viewers?

When creating pdfs with InDesign CS6 my target pdfs show sometimes strange patterns in the pdf. These patterns are sometimes black horizontal lines straight over the whole page, sometimes a square around a graphic I included, or some others. I experienced these patterns only on A4-pages which are completely filled up with one colour and a semi-transparent image (meaning only a part of the square-sized image is transparent) copied over it afterwards.

My problem is that I have problems with reproducing it. These patterns only show up when (in my current case) I am viewing the resulting pdf in Foxit PDF Reader at a specific size. When I zoom in, they are gone. When watching the pdf with the integrated Windows pdf reader, I can not see any lines regardless of the zoom level. Same goes for the integrated reader in Ubuntu. In InDesign I can not see these lines either. According to a friend he can see the lines when watching the pdf with the Adobe Reader.

Where do these lines come from? And how can I remove them? Or is it a simple bug of the viewers (Adobe InDesign -> Adobe Reader: Bug? Seriously?)?