Keeping track of - signs has always been the bane of my attempts at math. Somehow I always manage to lose track of at least one - sign during the course of working out a problem.
On one notable occasion working on a homework assignment for a boundary value problems class (partial differential equations galore), one question evolved into 13 pages of math scribblings. Eventually I decided it was far too ugly to be any kind of correct, so I pitched it all and started over. I found my mistake in a single - sign on the second page and wrote the solution up in 4 pages.
20 years later, things haven't changed. Still making the same mistakes with - signs or other silly little math errors. On the BioE 846 quiz I got back today, one of the easier questions I ended up getting wrong because of an off-by-1 mistake.
One positive: with the additional years of experience, I'm at least catching most of my stupid little mistakes earlier.