Small one:

I use tendu_default theme. Using it, if my content has a table (such as that created by fckeditor, but it doesn't matter, I think) with tbody element within, the table gets an ugly (=buggy IMHO) top border, which comes from system.css (modules/system/system.css), line 29.
I needed to put tbody { border: none} in style_subtheme.css to resolve this. I'm not sure if this should go into style.css, but I played it safe and put it in my subtheme CSS file.