For me, it was when i finally got to chose the Advanced Class for My Sith. I decided to go with Sith Assassin, and off I went on my merry way.
It took me about 10 levels until I realised that the strange bag that appeared in my inventory was full of the items I needed to become the Assassin....
This is awesome. My first character, for some reason I thought this was a token or something that we needed in our inventory so that our advanced class would "work". It just so happened that some 21 levels later, (yes, level 31 lol) I thought, hey let's leave this in my cargo hold and see what happens so that I can free up that extra inventory slot.
The very next thing I did was I tried to grab a +3 STR datacron from Nar Shaddaa (the crazy one with the pipe jumping). and it started coming up with error messages... "You must complete some missions before you can access this..." or something like that. So after having made it across those stupid pipes, couldn't get the datacron. I just knew it was that token that I took out of my inventory.
So I went back to the cargo hold and fetched it, then went back for the datacron, only then to find out that it wasn't the token at all, but my mission log was full. But still didn't put them together.
So I moved off to Tatooine to run missions after removing some of my mission log items, and just so happened to spot someone in General chat saying "Don't forget to open your advanced class bag."
At which point I felt like the dumbest player ever... and immediately right clicked it and found the custom assault cannon I was supposed to get back at level 10. LOL
Ahh... the noobishness...