I know that I'm often guilty of using the wrong fly on the wrong line. I tend to do it in early season or in high water conditions when I decide that I want to offer a larger fly to attract the trout's attention. So I tie on a larger streamer, even though I'm set up to fish medium sized wets or nymphs. Then comes the cussing from me as I botch cast after cast. What I should do in those instances is change fly lines. Someday I'll get smart.
Now, keep in mind that this chart, as good as it is, is only a basic guide. These are not hard rules. You can go over in fly size probably one to two hook sizes on each line listed without too much problem, depending on fishing conditions. Also, there's a little more gray area between species. I would not necessarily put a steelhead in with a 12" smallmouth bass. No, that smallie would go into the "Trout in most situations" category. So apply some common sense when using this chart & it will do the trick for you every time.
Thanks for the chart, Terryll.