Arguably this is a Fact of Life, but it might 
be fixable anyway.

Submissions including a literal < character not followed
by a literal > will "break" the HTML chatterbox: the 
part of the submission following the < will be 
swallowed, along with the <BR> tag that should follow 
it.


Example submission:
 bad submission ends here< instead of here


Result:

<OtherUser> random joke
<ChemBoy> bad submission ends
here<OtherUser> next message,
which is kind of messed up
<ChemBoy> Oops!


Apparently, something along the way is treating 

< instead of here <br>
 
as a single tag, which some subsequent step is removing 
as "not an allowed tag."  This gives the submitter (i.e. 
me) exactly what he deserves for not escaping his entities, 
but subjects everybody else to some needless confusion.