I've been using a dashcam for the past 2 years.
It automatically records videos and you can press s button to save the current video to a 'do not delete' folder where 3 minutes of the current video is kept and never deleted. Or os I thought.
Every few months we bring the dashcam home to switch the memory card and copy the files to an external HDD. This time though all the videos from easter to the end of may were gone!
Looking online it seems that if it's full it does not tell you, but instead starts deleting old saved and marked as don't delete videos at will. I've lost one month of videos including our Easter holiday because of this.
Mind, full does not mean that the card is full with saved videos only, it still had 32gb of space not occupied by saved videos, but instead of using that 'free space' it decided to delete saved videos.
I have no idea how a manufacturing company can make such blunders. I'm going to back it up once a week and I'm going to change manifacturer once it breaks or I find a particularly good deal.
It still way better than not having a dashcam at all, but having something that you can't trust to keep all recordings you save is not ideal.