When you are setting a default value for a table column, and when you are done, if you press enter thinking it would commit the default value, and close the popup box, instead it saves the table, leaving the default value drop down box still open. which can also cause an error if you have an invalid default value.
To duplicate this, you open up a table, and be on the Table tab, then make a change to the table, add a new column, rename one, anything that would then make the Save and Discard buttons active, then change the default value for one of the columns, and finally press the enter key on the keyboard.
The best solution would be to either disable the column screen buttons when the default value box is visible, or just to set the buttons for the popup as the default buttons