I think things that aren't suitable for freezing lose colour, texture, taste or a combination of all three, things that say don't freeze are because they might give you food poisoning ie things that cam frozen that you shouldn't thaw then refreeze.
....although i have frozen some things that say don't freeze, and they freeze pretty well. can't think of any right now (probably due to too much delicious italian red wine) but i defo have.
usually, those things which have already been frozen and shouldn't be frozen again are labelled as such as i think that's quite important.
maybe if you google 'freezing panna cotta' it'll tell you? in fact, i shall google it myself and get back to you :o)
Hey guys thanks for all your answers !
I know ive lashed things in the freezer before,defrosted and cooked them and not noticed until later but I didnt think of the 'previously frozen' angle-very good point and the curdling of the panna cotta once defosted so guess i'll just have to scoff them -lovely !!!