If you press . to display the autocomplete dropdown, then press Esc to close it, then use an arrow key the text cursor (or alternatively just use the mouse to move the text cursor while the dropdown is displayed) - a text cursor remains displayed as a non-flashing version at the . while the real one is free to move around. Getting rid of it requires switching to another window and back to force a complete refresh of the affected editor.
Happens in 2.2 and 3.0.
On Vista Home Premium with .NET 3.5 installed.
Cheers.