Yes, nothing good could come of that. I am all for over loading a rod, especially when casting heavy weighted lines, but by that I mean and 8 wt line on a 6/7 wt rod or a 9 wt line on an 8 wt rod, not an 8wt line on a 4/5 wt rod. Lines and rods are made to balance, the more balanced they are, the nicer the feel and less effort in the cast since the line is able to properly load and flex the rod. However, that being said, you do not need to stick to the manufacturers recommended line weight as there are plenty of situations that are going to call for a lighter or heavier line. A slightly heavier line can make that short cast in a small stream all the easier (since you only pick up a few feet of line outside the tip, the heavier line allows you to load the rod properly with less line out), just as a slightly lighter line can make that delicate cast on a calm interior lake land all the softer (allowing you to get away with a shorter leader). However, rarely is it necessary, let alone recommended, to overload by more than 2 line weights (an 8 wt line on a 6wt rod maybe, but never on a 4/5).