I just stumbled on to the solution to a problem I've had for quite a while. The only two symptoms of the problem that I noticed were that (1) when using the Sage Notebook, matrices of more than dimension two didn't display correctly, and (2) when logging on to sagenb.org I got a message along the lines of: It looks like jsMath failed to set up properly (error code -7). I will try to keep going, but it could get ugly. My various attempts to solve this were discussed in the sage-notebook thread Display of matrix in notebook.
It turns out that I simply didn't have the correct version of jsmath fonts installed. The answer on my Fedora system was:
yum install jsmath-fonts
The weird thing about this was that (as far as I could see) there were no other symptoms. So it simply never occurred to me that I was missing the correct fonts. Anyway, I just wanted to document this somewhere, in hopes that if anyone else runs across this issue they might possibly find the answer here. I'll also make a note in the thread mentioned above.