Currently Being ModeratedJun 17, 2013 5:43 AM (in response to Jim Bridger)
Happens all the time. The problem has been there for as long as I can remember, and shows itself in Keywords as well as Captions, maybe others. And yes, it is a major annoyance, and it is a bug IMO. You've found the same workaround I have, namely to restart Aperture.
If you haven't already done so, submit a bug report using Aperture Feedback to add your voice to the list.