Three things I have never managed to get used to, and that cause me to miss important events.
* September being 9 (and not 7 as the name indicates), October being 10 (and not 8), and same with November and December. Missed appointments because of this.
* 17h is 5 pm and not 7pm, 19h being 7pm and not 9pm. Missed planes due to this.
* the american month/day/year deal. Missed deadlines because of this.