i second that bit about q7. but you should also usually do 7 b) i) because it is usually as basic as the first question on the whole paper!
and as for stupid mistakes, I tend to make more on 2U papers than 3U, because i think it's gonna be more difficult in 3u, and spend more time, but in 2U I race thru it to get to the interesting questions at the end and I skip lines of working *slaps self on wrist* ...