I am a self-taught quilter and have made so many mistakes, that I learned from, and the major issue for me was that 1/4 inch perfect seam.
My research summary is that you should ALWAYS use a scant 1/4 inch, because the thread, the fold, the fabric weight, and the pressing enlarges that 1/4". The most popular way to do this that I had found was to use a 1/4" presser foot, with a fabric guide on it, and move your needle position once to the right.
If no fabric guide on your 1/4" foot, then I use the 1/2 inch sticky insulation tape to keep the fabric straight. It was all I had in the house and it worked great! As much as we strive to keep the fabric straight under the presser foot, it can shift that thread or two either way.
Pressing absolutely makes a difference. I press the seam as sewn to push the stitches into the fabric, then open the fabric and press to the dark, then on the wrong side, I press the seam flat again.
Alway square up your patches/blocks when making the first few or for the whole quilt. I like to find my issues as soon as possible. While I prefer to make all patches at once, I know know that making a few blocks first is more important for accuracy.
Ruler lines can make a difference too. Use the same ruler for all cuts and make sure that ruler is correct! If the ruler is a perfect 2", when you cut, you are still adding to the width if you are not right on the edge. For me, this means sometimes moving the 2" line inside the cut area, especially on my Olfa lip ruler.
I have yet to use templates, but I can understand how they could be a problem, between drawing lines, ruler lines, and the actual cut line.
So many teeny weeny details! I try to stick to rotary cut patterns to keep the sewing as simple as possible and easier to deal with the details.
I still screw up a block here and there! But what is a great homemade quilt without some fudging in it? LOL!
Diane