That's the thing, product id 13 and product id 14 are very similar, both with sizes and versions. But the warning only showed up for product 13, not product 14. I didn't try all products, but for all products I tried (all with size and version) there was no warning.
In addition, this site has been running for a few years without problems. I have to admit I haven't tested in a while, but it is our main product and we never had a complaint.
Maybe this one will remain a mystery :)