I also think it is a plot hole, but I won't say for sure until I've seen the movie again (and in English), I will say thought that I did think it kind of stupid that the left without the sword, what was the point of going without it? And why was it all covered up like a fossil and all the other swords weren't, well yes I understand that Lord Bern probably sat in the prison for a long time....but not THAT long or perhaps I missed something

There is another thing I didn't understand, what was Lord Bern doing in prison? Was he sitting there all that time???
And about the swords, its just a bad script!

good point De_De, why didn't Lord Bern get 'fed' to the green mist. Maybe he hid in the shadows that entire time and was overlooked, not sure what he ate though

Why would Aslan have given the seven swords to the seven lords if they Telmarines and didn't believe in Narnia? Why wasn't Coriakin angry at Lucy for endangering his Dufflepuds? Why didn't he made the Dawn Treader invisible if that would have protected them from the green mist?

The whole green mist plot really didn't help the plot at all, to say the least.

I think Coriakin sent the Dawn Treader crew to Dark Island to destroy the mist. He told them that they needed all seven swords, and expected them to search for them all before venturing to Dark Island. The only reason the Dawn Treader even sails in is because they assume the final sword is inside it, and need to retrieve it in order to destroy all of Dark Island.