@DoloresGrey I so feel you. And have made that mistake more than once - if it is an apartment or something not on the gallery I want to scream. I actually did it when I moved Jessica out of her house and almost left her boys with no furniture (thankfully I had a gallery copy).
@Melfox we are all noobs when new packs come out and were all once noobs when we started. There are so many mistakes to be made - I know I haven't finished making them all.
My two worst mistakes - when I first started and was trying to age up a toddler with a birthday cake, I accidentally aged up his father who was a young adult (twice!) into an elder. The child was still a toddler. I then tried to age the toddler with the mother but aged her too. I couldn't get it into my head that I was doing it wrong and kept doing it wrong. (Thankfully I decided to read the forums after that.)
My 2nd mistake - at a party my sim had her whole family there. She had been having marriage issues because another sim had flirted with her (despite her rejecting him) and the husband was very jealous. I used flirty music and paintings to get her husband in the mood to repair their relationship. I accidentally clicked on their young adult son for the romantic interaction (sims too close together). Due to the romantic auras and sims being sims - he swung his mother over in a passionate kiss. It caused his fiance to burst into tears and leave saying never call me again. And the father slapped the mother in the face and asked for a divorce. The mother and son kept making out. It was horrible and sad. I left without saving - no long term harm done but I lost a few hours of gameplay because I hadn't saved for a while.