diff -r 6651923ea3ac roundup/hyperdb.py
--- a/roundup/hyperdb.py	Wed Aug 20 12:13:49 2014 +0300
+++ b/roundup/hyperdb.py	Mon Sep 01 00:58:51 2014 +0000
@@ -232,8 +232,11 @@
                 try:
                     curvalue.remove(itemid)
                 except ValueError:
-                    raise HyperdbValueError, _('property %s: %r is not ' \
-                        'currently an element')%(propname, item)
+                    # This can occur if the edit adding the element
+                    # produced an error, so the form has it in the
+                    # "no selection" choice but it's not set in the
+                    # database.
+                    pass
             else:
                 newvalue.append(itemid)
                 if itemid not in curvalue: