stud, there are members here who can answer this much better than I can. I'm a little ashamed to say that I haven't been a good student of varieties and errors. I just do the basics and that's not a very good answer. But I want to answer you here, even at the risk of exposing my shortcomings.
A few years back I read a lot of literature about pocket change and bought the Srike it Rich book. That book wasn't worth the money, I felt. There's so much literature to be had for free. But I was actively looking at change, still am, especially the
State Quarter series and Lincoln pennies. In all change, I look for the basics: doubling, overstrikes, mules, mintmarks, silver
War Nickels, wheaties, etc., and anything that looks odd and out of sorts. But I have to say that in several-years' time, I've been rather unlucky or not very good at identifying things like small-date Lincolns. I keep looking for the above, however, and silver, of course.
I've had the most luck in finding Roosevelt and Mercury silver dimes and Franklin and silver
Kennedy halves by looking at my change, asking bank tellers about their half-dollar and dollar coins, and by checking Coin Star machines every time I'm near one. I like the bicentennial quarters and halves you can find in change or at the bank, though if they're not silver, I guess there's only a sentimental and aesthetic benefit to them.
I'm remembering some finds as I write this. I did find a New Jersey
State Quarter with a missing letter and I will have to try an dig that out to see which letter. That was exciting. And there's another
State Quarter I have--North Carolina--with a reverse that looks like it was struck in several directions. I'm sorry to sound so dumb here, but I stopped collecting for awhile and stopped thinking about all of this. I started collecting again early this year.