mgatten on "Variation SKU must be unique -- but it is!"

I'm using WooCommerce 2.5.5.

We have 183 variations across 17 products. Most of them are just fine, but about 20 of my SKUs are being rejected. All the SKUs are 12 digit numbers (our UPC barcode numbers).

When I try to create a variant with about 20 of our SKUs (748252302748, for instance) I get an error saying "Variation SKU must be unique."

I verified that the post_meta table contains NO instances of 748252302748. Then, just for good measure, I performed a database-wide search of every field in every table, and the string "748252302748" appears NOWHERE in the databaase.

I found a report of another person having a similar problem, and they optimized their database and that fixed it. So I tried that, but to no avail.

I also assigned a test SKU to the variant, and that worked, but when I tried to change it to 748252302748 the error came back. I checked the post_meta table again, and then checked the entire database again, and the value "748252302748" still did not exist anywhere.

There are about twenty different SKU numbers having this problem. All are unique, and none of them appear in the database.

We have another WordPress/WooCommerce site running on the same versions, where all 183 UPC are working fine.

Any ideas?