This will tend to happen intermittently sometimes.

When it loses power, the unit uses the last dying gasp of electricity to quickly save it current position into flash. Sometimes it's not quick enough and there's not enough juice to keep it alive while it does it.

If it only happens once in a while, it's normal. There's nothing that can be done about it. Personally, I think it's amazing that it can do it at all.

However, if it happens every time you shut off the ignition in the car, it's a known problem, covered by this FAQ entry.

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Tony Fabris