The game client renders fleets in the zoomed in tactical interface even while they are still in transit to a world. For starship fleets which travel at 0.2LY/watch, they are visible (and targetable) in the planet view for a few minutes before they actually arrive, and their status is still "in transit".
This is problematic because players can order an attack on these fleets, which do not fire back due to the "in transit" status effect. Essentially, any starship fleet a few minutes away from arrival can be destroyed for "Free" by a player who has a fleet in orbit of their destination world.
This was discovered in a late Era 3 war involving TheBugKing and myself against Wayward Device, both of whom can therefore confirm the existence of the bug/exploit.
It is partially fixed if Era 4 starship speeds will be increased to 1LY/watch as suggested by some in the forums, however the underlying issue will still be present and will affect any future slow-moving ships/fleets (constructions?)