I'm such a dill sometimes, I'm pretty sure I've had a stir-bar in the fermenter about half the time if I use a starter. I just seem to forget its there, swirl it up and dump the lot in. I keep extras now, so I'm not without my stir plate waiting for the beer to finish so I can strain it out of the trub.
I haven't lost one down the sink (they are usually in the fermenter!)