This can only happen if numerous files are marked as “protected”.  A protected file is permanently saved and will not be deleted.  If enough protected files accumulate, it could eventually fill up all available memory.

There are only three ways to protect a file:

If you press the orange button will video is recording, then the current video clip will be marked as protected. This feature only protects the current video file. If you access the menu from Playback Mode, there is an option to protect files. If the G-sensor detects an impact, then the current video file will be automatically protected.

What to do:

Empty the memory card.  Refer to the FAQ “The memory card is full”. Avoid manually marking a file as protected unless you really need to save one. If the memory still fills up, then the G-sensor might be triggering due to excessive vibration.  Try reducing the sensitivity or disable the G-sensor.  Refer to the FAQ regarding “Video menu options”.