I always make HST using larger squares figuring I can always trim. Since I had done a lot of sewing, I knew how to handle bias (think A-line skirts, etc). However, I prefer not to handle bias on the outside of a block -- especially since I can't finish piecing a quilt at one sitting and I sometimes forget I am handling bias edges and then I am in trouble (sigh). If you do have to work with bias edges -- as a lot of folks have said -- STARCH!!!!
Before I start sewing, I always double check my 1/4" seam -- different fabrics take up different amounts of fabric when the seams are pressed (some fabrics are thicker than others -- and not just flannels). I always measure the first pieced block and adjust accordingly -- as was mentioned earlier, being off by just a bit every time a seam is sewn can result in being off by a lot. Here is some math to think about -- if a seam is off by 1/32" on every seam then after 8 seams the block can be off by 1/4". If you measure as you do the first block, you know where things go off and you can be more careful there.
Another thing that can send a block off is when you press the seams -- if you are not real careful you may wind up with a fold instead of a press seam -- this can be as much as 1/8" (ask me how I know). Talk about throwing a block off!!
I think I have made about every mistake possible piecing -- sometimes more than once but then I think I may be a slow learner.