It's a safe bet that as big as D&D is, as many rules there are, classes and feats to choose from, it's going to take a while to get used to it all. Everyone's bound to screw up at some point. Your character died because you forgot to add your strength modifier to damage and didn't kill the big baddie before he could kill you, you forgot to increase your spells per day for a few levels, I'm interested in hearing what your story is for your biggest gaming goof-up. Post in the comments, or email us at firstname.lastname@example.org Don't be shy. We don't bite... often...
For me, my biggest mistake was about character growth with my first character. She was a CG half-elf rogue named Ranel. I'd made her for another game before I started college and joined my first group. She'd really only been played once, and I wanted to use her again when I joined DM's game. I started around level 7 or 8, I believe, and Ranel was originally very low level. With DM's help, I upgraded her to the proper level and joined in the game.
A few levels later, I was getting worried about her. Our game was somewhat combat heavy, and I was afraid she wasn't strong enough to survive the combats much longer. I think I was spending a good amount of time hiding versus fighting. DM took one look at my character sheet and was absolutely shocked! I remember he even later commented he was amazed I'd survived that long in the first place. Newbie me had completely forgotten to add my Constitution to my HP each level! I was just taking the base roll of her d6! I occasionally still do this. I may have also been forgetting to add my feats and ability score points as I leveled. Basically, if it wasn't explicitly stated in the Rogue section of the player's handbook, I screwed the pooch leveling her up. After that, DM had to sit down with me a second time just to get her back up to normal. I'm just glad my major newbie mistake didn't end up biting me in the butt and costing me a character that I ended up taking to epic. That would have sucked.
How about you, Ravi?
Incidentally, my biggest mistake I ever made was actually WITH Ravi... and could have cost me the character, had the rules of the fight been different. I was fighting a demon until first blood (half hit points), and I was actually doing okay... he still won, though. The next fight, I realized I forgot to add my Favored Enemy: Evil Outsider bonus to my damage rolls... a whopping +6 PER HIT. I felt like an idiot.