This can be a legitimate error if multiple users are trying to edit the same item. However, it appears as if it may be happening due to a flaw in the caching mechanism whereby the serial number is updated twice or something to that effect. Here is a rather old thread where the issue is discussed: http://gallery.menalto.com/node/56767.

See below for a representative trace.

Error (ERROR_OBSOLETE_DATA) : UPDATE g2_Entity SET g_modificationTimestamp=?,g_serialNumber=? WHERE g_id=? AND g_serialNumber=? (1263304960|10|4b455657d2bbf|9) in modules/core/classes/GalleryStorage/GalleryStorageExtras.class at line 396 (GalleryCoreApi::error)
in modules/core/classes/GalleryStorage.class at line 324 (GalleryStorageExtras::saveEntity)
in modules/core/classes/GalleryEntity.class at line 293 (GalleryStorage::saveEntity)
in modules/core/classes/GalleryItem.class at line 417 (GalleryEntity::save)
in modules/core/classes/GalleryAlbumItem.class at line 256 (GalleryItem::save)
in modules/core/ItemMove.inc at line 171 (GalleryAlbumItem::save)
in main.php at line 317 (ItemMoveController::handleRequest)
in main.php at line 124
in main.php at line 89

Request variables: Array
(
[return] => /ccs/main.php?g2_itemId=4b4554bf2b688&g2_typeView=GridView
[formUrl] => /ccs/main.php?g2_view=core.ItemAdmin&g2_subView=core.ItemMove&g2_itemId=4b4554bf2b688
[authToken] => 6ee9d7bff3c2
[controller] => core.ItemMove
[form] => Array
(
[formName] => ItemMove
[reset_permissions] => 1
[selectedIds] => Array
(
[4b455657d2bbf] => on
)

[destination] => 4b4c809dc558a
[numPerPage] => 15
[action] => Array
(
[move] => Move
)

)

[itemId] => 4b4554bf2b688
[page] => 1
)