After I modify and re-evaluate an expression, the new result doesn't replace the old one. Rather, what happens is that the old result gets pushed down and Mathematica displays the current result at the top. It looks like this:
In := (* expression *) Out = (* result *) Out = (* older result, when In was In *) Out = ... ...etc
I'm pretty sure I messed up my preferences, but I can't revert to defaults, because I have another issure resolved (after trying for some time) and not sure exactly how I did it. Following old
If It Works, Don't Fix It
So I'm reluctant to restore the default settings, the other (fixed) issue being far more important to me.
(Mathematica 9 on Ubuntu 12)