see: http://drupal.org/node/300084

Anything dragged to the very top will not stick.
This happens because siblingWeightFieldUp[0] ends up as undefined and then javascript stops processing the rest of the code because the siblingWeightFieldUp[0].value fails.


diff -Nurp ../cck.orig/modules/fieldgroup/fieldgroup.tabledrag.js ./modules/fieldgroup/fieldgroup.tabledrag.js
--- ../cck.orig/modules/fieldgroup/fieldgroup.tabledrag.js	2010-05-24 09:51:44.000000000 -0500
+++ ./modules/fieldgroup/fieldgroup.tabledrag.js	2010-05-24 09:51:54.000000000 -0500
@@ -53,9 +53,11 @@ Drupal.tableDrag.prototype.onDrop = func
   if (siblingIndent == indents) {
     //the previous row is a sibling, so let's set the new weight
     var siblingWeightFieldUp = $('input.field-weight', siblingRowUp);
-    var siblingWeightUp = siblingWeightFieldUp[0].value;
-    var newWeight = parseInt(siblingWeightUp) + 1;
-    $('input.field-weight', tableDragObject.rowObject.element).val(newWeight);
+    if (siblingWeightFieldUp[0] != undefined){
+      var siblingWeightUp = siblingWeightFieldUp[0].value;
+      var newWeight = parseInt(siblingWeightUp) + 1;
+      $('input.field-weight', tableDragObject.rowObject.element).val(newWeight);
+    }
 
     //now deal with the case where we moved left/right
     if (oldClass != newClass) {