Well, I thought I had this resolved, but it appears to still be happening. To recap, items in treeview1 are dragged over to items in treeview2. I can step through the code and see that the oledragdrop event of treeview2 has finished running cleanly and exited the event. If all items in treeview1 have been dragged and dropped to treeview2 then I close the form for the user. This is the same process as when the user clicks on a button to move an item from treeview1 to treeview2 to process it (see the call to cmdadd below). Treeview1 is cleared and repopulated afterwards and if there are no items in treeview1 then I display a message (using messagebox) and close the form for the user. Everything runs cleanly when the user clicks on the button themselves to do the move but when the user uses oledragdrop if I close the form afterwards for the user then after the form is destroyed and the calling form is reactivated, a C5 occurs. If I step through it I can see that the oledragdrop event below has finished running.

LPARAMETERS data, effect, button, shift, x, y
DODEFAULT()
LOCAL oNode
oNode=this.Hittest(x/96*1440,y/96*1440)

IF TYPE("oNode")="O" AND !ISNULL(oNode)
	*--Selected a node on the right (policy or name)
	this.DropHighlight = oNode
	IF !EMPTY(thisform.tpolicy)
		THIS.nodeclick(onode)
		RETURN thisform.cmdmove.click()
	ENDIF
ELSE
	*--Did not select a policy or name on the right (no node selection)
	IF !EMPTY(thisform.tpolicy)
		RETURN thisform.cmdadd.click()  && see note below!!
	ENDIF
ENDIF
*NOTE:  if the user clicks on the item in tree1 and then clicks on an item
*       in tree2 and clicks on the cmdadd button, everything runs fine and
*       no c5 error occurs.  The form only closes if all items in the tree1
*       have been processed and the same code is run when the user clicks
*       on cmdadd and when he/she uses the mouse to drag/drop.

There is only one other object that is created in the process run from the cmdadd button and it is a progress window. I check for it and release it if it exists before the form is released. I thought not releasing that object was causing my c5 error but it is still ocurring.