Save as you go !
It's an onbvious error I never thought I'd find my self making....
Whilst editing an activity there is a time limit for saving before you continue, due to security tokens I'm told. If this time limity is exceeded you will lose all the edits you have made. Failing to save as you go is a ludicrously obvious error, but it caught me out and wasted a lot of time.
I use Wordpress and left the browser open overnight on an activity I was editing. When I resumed the following day all seemed well, until I tried to add an image to a drag and drop activity. I got an invalid token error. Consulting this forum I decided to save the activity and try again. When the activity was re-opened I had lost ALL the previous edits, not just the one with the error.