This can happen in rare cases where more than one add-on is starting up at the same time. B1 Usability Package version built after March 2007, take special care of this, but if you see it with other add-ons it should be reported to both/all manufacture of the add-ons since this is an indication that SDK guidelines are not followed.
OR
This is a known issue in SBO 8.81 PL05 and PL06… SAP expect it to be fixed in PL07 or 08… See more here: http://www.boyum-it.com/forum/viewtopic.php?f=2&t=272