The Most Common Errors in Undergraduate Mathematics (2009)
Bad handwriting.At age 40 I started taking a masters in stats and had situations where I had exponents on exponents.  This lead me to buy a higher resolution tablet for reading pdf's with tiny math.I also bought a finer point pen and this helped me improve my handwriting a lot.  Closing loops on "o's" or backtracing the upward line of a cursive "t" to not make a loop.  With the finer point, I was able to see my imprecision and improve it.I used to rewrite my finished work in undergrad and now my writing was improved to the point where the first draft was entirely legible.