I have a weird one for you - I have an master/detail form and the detail region is an updateable tabular sql query.
I have a quantity column which is updateable, and I want to show the total. On the report attributes/ column attributes, I have checked the "sum" checkbox. This successfully shows the sum total of my quantity column.
However, if I specify a default value for the column, it ADDs this default value to the sum total.
I have modified the sample DB application to show total quantity on the Order detail and it demonstrates this "feature" : http://apex.oracle.com/pls/apex/f?p=19287:4 - log in as demo/demo and edit order 0010 to see that it adds 1 to the total quantity (which is the default value for quantity).
I raised this as an SR on MOS and the response from development is that it is not a bug but a "feature" - their action is to update the documentation to tell people not to do it (not even to stop APEX developers from using the combo) !
I love paying oodles of cash to Oracle in maintenance so that they move the goalposts when you try to get something fixed !