For me, 2 things stick out the most..
Not paying attention to form and technique.
Not paying attention to the big 3 lifts.
I didn't start benching with any seriousness until just a few years ago. Same with Squats and Deads. If I went to a gym, it was always curls, leg presses, and tricep pushdowns.
Oh, never enough cardio. I still hate it now, but I know if it was something I did with routine, I'd be in much better shape, both cardio and muscle wise.
Mt Snow, VT - May 5, 2012 - Completed!
Gunstock, NH - June 1, 2013 - Completed!
...i remain, he who remains to be...