ForumNG caches discussions in session in order to make the 'Expand' links work faster. Unfortunately this causes problems if people write very large posts (e.g. we have a case where somebody did a massive copy/paste from Word, so they have a 2MB forum post, of which probably about 1.9MB is repeated crap from word, but whatever). When using certain mechanisms for session storage, there might be a size limit which this exceeds.

In order to address this I have a dual approach:

1) Reduce max number of cached discussions from 5 to 2. (Maybe slows it for people who use lots of tabs but they probably used more than 5 already.)
2) Do not cache discussions in which the total size of all 'message' fields is larger than 200KB