lets face it. he made loads of mistakes, especially with the horcuxes
he told slughorn in book 6 that the number 7 was the most powerfully magical number
so he decided to make 7 horcruxes, with the 7th bit in his body
1. the ring
2. the cup
3. the locket
4. the snake
5. the bit on his body
6. the diary
7. the tiara
then dumdumdummm
8. the bit in harrys body
messed up again....loser.