Hi nick258,
I didn`t find any bug with planet deletion.
There is several checks to prevent unwanted deletion. All planets has unique id, when you confirm deletion this id sends to server and compares with id of your current active planet (in case you switched planets in other tab or browser).
So to delete planet:
1. planet to delete need to be active planet on current tab/page of browser
2. planet to delete need to be active planet on server (in case you switched planet in other tab/page)
3. your credentials are right
4. you confirmed deletion
We think this is enough to prove users intentions and prevent unwanted deletions.
Also we changed rules about fleets. Now you cannot leave planet if you have any fleets flying TO that planet, any other flying ignores.
Maybe you tried to delete this planet but expected error as you have fleet in transit.