I've just found out that Slider's fix posted on a previous page in this thread about detpacks and siege objectives when a player changes teams/disconnects does not entirely work. I've tested it and it still seems to cause them to damage the objectives.
Better solution: Remove all detpacks, trip mines, and thermals from the player who switches teams or disconnects/reconnects.
Also: BugFix13 is only part of the problem with the siege bridge lame, a properly placed player or other objects can still block the bridge and make it go back in. Fix: in SP_func_door make ent->spawnflags be 5 (start_open and crusher) for that mover and set ent->damage to 9999 if it is on mp/siege_hoth and is func_door and has an ent->target of "droptheclip".