I'm using Netscape 4.78 here at work, and as far as I can tell, LiveJournal is the only web site this is happening with. Bulletin boards I post to elsewhere don't have this problem. Also, this problem has only been happening in the recent past--I want to say post-UTF-8.
I know it's not a problem with the entry itself, since I can go home and edit it to add the last character fine. It has to be something with the way Netscape 4.78 is formulating its headers or with the way LiveJournal is parsing them. Looking at update.bml, more than likely it's a problem in bmlp.pl or somewhere prior in the execution tree. Something with language parsing.
Yes, I've set my default language to English. Netscape is also configured to use "en" as it's first (and only) language choice. If there's anything else that I can try here to give more debugging information, just let me know and I'll see what I can do.
Edit: It doesn't do it to comments. Just entry posting and editing. Maybe it's something to do with the postevent method in ljprotocal.pl.
Edit Again: If I manually type in a command to /interface/flat in the URL bar on my browser, it does NOT clip off the last character. Something in the header's on the BML pages might be confusing Netscape?