I have noticed that an empty tr is inserted at the end of a month in calendar view. This causes the bottom border of some of the cells in the last "real" row not to display (in earlier versions of Firefox, anyway…haven't looked at IE yet…). Right now, I'm using jQuery to help.
It would be better if that blank row were just not there to begin with, obviously. The site I'm currently working on has this problem, but is not live site. I found EM because the client requested it after looking at this site, which also exhibits the problem: http://mainbeach.com/events/