What happened: I placed down my cannon in the glacor cave. Tele'd out after a while to bank and left the cannon there. Came back to pick it up and nothing happened. Reload option gave me a "You are not the owner of this cannon". I relogged, had the cannon in my inventory, but it was also still on the ground.
To try and remove it, i placed it down again and the same thing happened. I did not teleport out this time though. All cannons in the included screenshot are (technically) mine.
How to re-create: Simply place down a cannon and try to pick it back up again. If this does not work, try placing it down and teleporting out and back in.