In request #12985, it was pointed out that some LJ code (most of it, actually), is outputting plain "&" characters instead of the HTML entity "&". Is this a major concern? It does violate the HTML 4 spec.
I went through the code and fixed each use of "&" that I could find; the patch is here. I have not tested it, so I'll leave it to the rest of lj_dev to decide if it's worthwhile and/or safe to be applied.