I just got the 1957/DEC13 printing of the core set. Is that the latest one with all the errata and easy mode cards?

You can check by just looking if the encounter cards have rings around the set icon.

At the moment it's sealed, and if I open the seal I can't take the box back to the shop.

TriumphantOgre: Go ahead and open it up. That is the most recent, up-to-date Core set with errata and easy mode rings and such. Just look on The Grey Company Podcast's facebook page for pictures of the barcode if you want reassurance. DEC 13 it should read.

Brilliant, thanks Kyrie. I'll take the box home tomorrow.