This is empirical, but if you add a note, click enter (i.e. the "Add" button) and then refresh IE (e.g. by pressing F5) while still in the answer window, the note will appear again.
This isn't much help, but it simply happens sometimes.
[Edited at 2005-03-01 20:33]
Thank you both for your replies. I'll take care not to do that in future!