This is not quite reproducible, but it has happened a few times:
If I start typing a name (e.g. "debug"), then partway through the list of completions pops up. If I'm typing very slowly, this happens after "deb". Sometimes, not always, if I continue typing because I didn't notice the popup, something weird happens: for this example, the "u" appears after "deb" in the console window, but the cursor is positioned before it. When I type the "g", it is inserted in the wrong place, so I end up with "debgu".
Once this happens once, it is easily reproducible in that session. However, I'm not sure what triggers it in the first place.