Yep, they can be wrong. I had my dating scan at exactly 7 weeks. It measured bub at 5+6 with a heartbeat of 142.
I had my midwife appointment at 12 weeks and she said that it wouldn't be an accurate sizing given the heart rate and disregarded the dating scan. We decided to run with dates until my nt scan.
Not scan measured bub exactly on dates at 13+1. Due date is at the original date as per my cycles.