QT Subtitles - "Movie contains an incorrect duration"?

[Chris Searles]

I'm trying to add subtitles to a QT movie I made (following the 
instructions in Erica Sadun's book) and after a first successful try I 
made a couple of adjustments to the beginning and end times of a couple 
of titles and since then I keep getting the following message:

"Couldn't open the ... file because the movie contains an incorrect 
duration"

Could someone please explain the most likely cause for this and what 
I'm probably overlooking. It's driving me batty!

Thanks.