We have installed 20 Raspberry Pi systems in a room where they are used by kids aged 13-18 that cannot buy their own computer. The systems were working perfectly, but since around two weeks they cannot save data anymore.
What happens is when you save a file on the SD card it looks like it has been saved. However, when you reboot the Pi, the file simply is not there anymore. There is sufficient free space, the cards are 32GB and only around 8GB is used. Even if you don't reboot, after around 20 minutes the file will also disappear.
This happened to all devices at the same time. It is really strange.