i'm sure with so many h/c members if they all decided too post their experiences you'd have a list a mile long of trading mistakes.
my gut feeling, many are just trigger happy (hit the buy button) without really doing any reseach (be it technical/fundamental).
self discipline is probably in my top 3.
for my daytrading/short term trading i went out bought some trading software and haven't looked back. so it does pay to spend a little money to reap the rewards, success doesn't come for free.