Now that new versions of snapcraft are widely available (ie, 2.40 is in the snap’s stable channel and -updates for all Ubuntu releases (which LP and build.snapcraft.io pulls in automatically)), as of yesterday (2018/04/12) the review tools in the store now require the snap is created with
-no-fragments (which snapcraft does as of 2.38). If you are using an older version of snapcraft to build the snap, you may see snaps fail review with:
Processing...| Error while processing... The store was unable to accept this snap. - checksums do not match. Please ensure the snap is created with either 'snapcraft pack <DIR>' or 'mksquashfs <dir> <snap> -noappend -comp xz -all-root -no-xattrs -no-fragments'
To resolve, simply upgrade to a newer snapcraft, rebuild and push to the store.
Based on input from @evan, I’m in the process of improving the error message to mention 2.38.