I always think of "rookie mistakes" as simple mistakes that seem larger than they are to an inexperienced person. But I've played The Sims since the very beginning, so I'm rather experienced. So when I got the Sims 3, I didn't make simple mistakes...I made very complex mistakes.
I knew all about the sim's moods and how it affected them in their daily life. But I was utterly clueless about moodlets and the impact that those had on a sim's mood. I'd get all the bad ones: poor night's sleep, poorly cooked food/fridge flavor, negative socializations...you name it, my first sims ended up inflicted with them. It took them forever to get promotions, and their mood was always low despite the fact that I was struggling like mad to keep their needs met. It was driving me crazy lol.
Another very complex mistake was that I bought my first couple the "fertility treatment" LTR as soon as they had the happiness points for it. At the time I didn't really realize how easy it was to actually get a sim pregnant, so I thought "well, if it exists, there's probably a reason for it." And for some reason, they just seemed to adore children's radio (and they weren't even childish). You can't imagine the stress triplets caused me as my very first sim procreation. Chaos reigned in that household until the first generation started school, let me tell you.