Give some thought to naming the waypoint missions you save in your favourite flight planner.
My poor little Mavic Air was given Bridge 3 as a task, not the intended Rail Bridge 3. When I saw it rapidly escaping in the wrong direction it dawned on me something was amiss …
You’re lucky you were able to cancel the mission while it was still within comms range 
Yes, very lucky. I’ll post the video later but the basic error is that Hayling is served by a road bridge which was my target yesterday and once had a rail line which also had a bridge. Todays mission was to video that. Waypoint ! for both missions was 20m altitude as it was over water and rising progressively at each waypoint. The two launch sites, yesterday and today, ar about 300m apart.
So the Mavic Air happily went off at 90 degrees from the intended track to waypoint ! of yesterday’s mission. 20 metres height, skirting large trees and over a busy main road. Once I realised what it had thought I had wanted it to do I let it complete part of yesterday’s mission before giving the Oi! Come here, bad drone! command. It came back quite happily, having risen to 40m safety height first.
If I had let it carry on it would probably have been perfectly alright though I;m not sure whether the battery would have coped with the extra distance the extended mission involved. Could have been a lot worse - if I had mistakenly put in a different mission it might have flown five miles south or 100 miles to Gloucestershire to get to a waypoint 1 