Destroying a permanent with indestructible should give a warning
When I use Poison the Cup on a creature with indestructible, it just lets me target that creature without asking if I'm sure. This is very annoying since it's easy to miss that a creature is indestructible.
When I try to do other things that are obviously stupid, like countering a spell that can't be countered or destroying my own creature, it gives me a warning asking "Are you sure?". It should also do that when I try to destroy a creature with indestructible.