Mismatch in cells border management
How to reproduce ? please see the included .gnumeric file and related snaphots.
1/ Select B2 and mark a border on top/bottom/left/right 2/ Select D2 and mark a border on top/bottom/left/right 3/ Select D1 and remove its bottom border - which it D2 top's one 4/ Select D1 and mark a bottom border
Actually it looks like the way it is stored in the xml file is as following : For two adjacent cells A & B, the border between A & B may be described both for A and B. Actually it is only for the one which was edited for setting its border. Hence, in the example the border between D1 and D2 is eventually only described as a property of D1 border_diff.xml
It might be only internals and not wrong per se but when used for instance to print or copy and paste to third party applications, imo the border should be "normalized".